This is completely incorrect. Bless is a +1 to your AB which does not necessarily translate into 5% increase chance to hit.
If your enemy has an AC of 15, and you have an AB of 6, adding the roll gives you a range of 7-26, which means you have a 45% chance to hit. Bless adding to this means you now have a 50% chance to hit the enemy, but your chance to hit has actually increased by 11.11%. If you previously only had a 5% chance to hit, bless would actually increase your chance to hit by 100%.
Now bless is a fairly minor spell but that +1 AB is always relevant because numbers in DnD are small and not bloated. Anytime you find an enemy that you actually miss against fairly often is always a good opportunity to use a bless spell. On top of this, the spell as opposed to more powerful buffs lasts a long bloody time. I recall in IWD2 being able to clear maps before my bless ran out. There is almost no excuse not to cast it all the time.
Honestly, pretty fucking awesome for something you get at level 1.