For the bless weapon spell...
All critical rolls against evil foes are automatically successful (so every threat is a critical hit). The weapon negates the damage reduction of evil creatures and is capable of striking evil incorporeal creatures as if it had a +1 enhancement bonus
Any weapon that has bless weapon cast on it, or that has magic weapon cast on it by a good cleric, is also blessed, as below.
Now, for a blessed weapon:
For instance, the gelugon and pit fiend do not regenerate against damage inflicted by a blessed weapon.
The rakshasha, as noted by another member.
There are others, but off-hand, I can't recall these other monsters.
The special ability against certain monsters varies by monster. The MM was written after the PHB, so this is one reason why those abilities aren't listed. Also, they knew more monsters would be published later.
You can't cast bless on a weapon, but you can cast bless weapon on a weapon, and you can also cast magic weapon on a weapon, so it's considered blessed. These are all different things.
Much confusion could have been avoided if bless weapon was given another name.