[QUOTE=Ashgan]I'm going to play devil's advocate here if you'll indulge me, as I cannot really overcome my bias against blood echoes. The first point I want to make is that Rom's defeat is a game state trigger for a metric ton of things. The Doll's voice line being just one of them, I don't know that we can necessarily say that it is directly aimed at making commentary on Rom's blood echoes. It's easily imaginable that the designers figured "Okay, it's the halfway point in the game, player probably has a bunch of blood echoes by now. Seems appropriate to put the line in here", and they used Rom's death as the trigger to make it appear. In other words, the Doll doesn't so much say it in response to Rom's death as she does to the passage of time that has elapsed so far. To support my point, I would also point toward how killing Gascoigne makes the sun descend, and how killing Amelia turns dusk into night. Surely nobody would argue that it is their deaths that caused this, but simply a gameplay contrivance to advance the time of day in relation to the player's progress in the game. In a similar vein, I think Rom's death is simply the trigger for a bunch of new things in the game. Maybe I'm selling From short but for all their genius in world building, I think they frequently do make gameplay concessions; Souls titles are not story-driven games, so I feel that much of the things we are presented with have to be taken with a grain of salt.[/QUOTE]
I will start out by saying that, in theorizing about Bloodborne (and a lot of other games, for that matter), I try to avoid the meta-angle as much as possible due to how [I]insanely difficult[/I] it makes theorizing. If I were to comment on the meta-reality of blood echoes, yes, I fully agree that their presence in Bloodborne is most likely just to mimic souls from Dark Souls mechanically (which, in turn, mimicked souls from Demon's Souls; I'd argue that if we get technical, souls (aside from Lord Souls and the like, which actually do have lore significance) didn't make sense contextually in Dark Souls either and most likely only were there because they were in Demon's Souls and still sort of made sense thematically in Dark Souls).
In isolation I would probably dismiss blood echoes in a heartbeat, but they don't exist in isolation, and dismissing them would create a problematic precedence. If blood echoes aren't real, but just there for the sake of mechanical gameplay, what else can we just discard on that basis? Parrying in Bloodborne, I'd say, is probably something added solely for the sake of gameplay. The follow-up visceral attack (that is also doable in other, non-parry situations) might have/most likely has lore implications, but parrying with guns is almost certainly just for gameplay. Bloodtinge, too, probably just exists to have a stat for blood-damage to mechanically scale with. Arcane? Doesn't make a lick of sense.
And once we start, where do we stop? People have spent huge amounts of time pouring over the details of visual design of so many things in the game, down to particular similarities in design and recurrence of certain colors, trying to find meaning in it all. Who says it means anything, though? Could just be to clue in the player of certain behaviors and strengths/weaknesses, or just to make things look cool/scary/disturbing. Everything but specifically written-down lore comes into question and different theorists can pretty much just pick and choose what they want to take into consideration, because anything can be there solely for gameplay or aesthetic reasons.
Considering things I know are most likely there just for the game rather than being "true canon" is a choice, yes, but not one I've made idly. It's a matter of framing one's assumptions, because if you alter the frame for one particular thing, you have to alter it for everything... that, or you pretty much make the lore-equivalent to invoking deus ex machina to resolve narrative problems you don't know what to do with. To me it's everything or nothing.
Likewise, saying "killing Rom is the trigger for lots of things, so they probably just did it because it felt like a convenient time" is the same sort of selective reasoning, hand-waving the situation because... well, honestly it's probably true, at least partially. But once again, once we start, when do we stop?
There's also just the matter of Rom's death generally just being that much more significant an event than anything else in the game up to that point. With Gascoigne and Amelia, yes, I fully agree, the changing of the time of day was almost certainly just as a convenient demonstration/reminder of the passage of time. What happens after killing Rom, however, I just can't accept as being nothing but it getting a bit later in the night. Too many things happen too suddenly and too [I]unnaturally[/I]. Besides, we pretty much witness the blood moon being called after the fight; it doesn't happen on its own, but is actively being caused by someone or something.

[QUOTE=Ashgan]I also wonder what the line is like in the original Japanese script, and whether it would have a different implication.[/QUOTE]
I undertook the search for this and actually managed to find a quite interesting [url=https://docs.google.com/spreadsheets/d/1NKkhAVz338xavCMiHplN--qxQG63K5kpBFDsbNUlg5g/edit#gid=0]compilation of retranslations[/url] from the game. Here is the translation of the Doll's equivalent line in Japanese:
[QUOTE=The Doll (Japanese)]"Hunter-sama... I feel *familiarity coming from you... It's because Hunters are inheritors of the old **wills as I expected..."
"*懐かしさ doesn't translate well. It's a fondness for something in the past, like nostalgia.
**意志 - ishi - Oddly enough, this isn't the same ishi used to describe blood echoes. This form of ishi is used to describe volition, will, etc."[/QUOTE]
So it does actually seem to reference something else than blood echoes, but also reinforces my assumption that there is something [I]more[/I] at play here. "Inheritors of the old wills"... Ah, can mean many things. I would have said that it could be the will of past Hunters, but she seems to reference that Hunters collectively are inheritors of these old wills, so I don't know... It still feels like it references some kind of originator or progenitor. Interesting.

As for dogs and crows (and animals in general, I guess), no, they almost certainly haven't received blood treatment... but it's probably not too outlandish to figure that they could have gotten Old Blood, and possibly associated blood echoes, from feeding on corpses.
Will think on it some more, but generally take the details of my prior theory with a grain of salt; it was pretty much a flow of consciousness-kind of thing. I practically made it up as I went. I'll most likely change my mind about some things as well in time, it was just what I thought of in the moment.