[quote]
[h3]Bless[/h3]
1st-level enchantment

Casting Time: 1 action

Range: 30 feet

Components: V, S, M (a sprinkling of holy water)

Duration: Concentration, up to 1 minute

You bless up to three creatures of your choice within range. Whenever a target makes an attack roll or a saving throw before the spell ends, the target can roll a d4 and add the number rolled to the attack roll or saving throw.

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, you can target one additional creature for each slot level above 1st. [/quote]

I pulled this description from [url=http://engl393-dnd5th.wikia.com/wiki/Bless]here[/url] so it might be wrong, but I see nothing that says it can only be done on one attack or saving throw roll. Hypothetically speaking if you bless someone who can do extra attacks, you can use your bless for each one of those attacks.

I can kinda see how your GM might have thought this was too powerful [@Norschtalen], if you can interpret it that way I just mentioned. This is effectively giving someone a minute-long +1 (At a minimum) to their attacks and saves, which oddly enough is good at both low and mid level. At low level the DCs and ACs are generally lower, so one or two points can make a big difference between a hit or miss. Even at mid level like 5 or 6, that's when martial classes would start getting their extra attacks, or some classes start to get really crazy like two-weapon fighters or monks. Usually those times gain multiple attacks for less damage/accuracy, but with a bless it can help offset that penalty.

Even at higher levels a +1d4 to a save can be really helpful considering how deadly those things can be. The only problem is that by the time you get to that level, there are plenty of other spells that compete with the strength of Bless. I don't know what spells since I've never played a cleric, but I'm sure there is one. So really it's not as if Bless is a bad spell later on, quite the contrary: it's very good, it's just that it has a lot more competition from equally as useful higher level spells.