Actually casting Bless Weapon after GMW doesn't override the higher enchantment bonus; Not at least on my paladin, granted his GMW is +4.
sounds like a bug/exploit to me
It's intended if you refer to stacking. Even before Bless Weapon was changed to On Hit damage (so that it can be used against undead PCs) these two stacked.
On the flipside, after that change Bless Weapon no longer stacked with alchemical varnishes, Flame Weapon, nor Darkfire.
Now that used to be
fun, run around with sword glowing of Bless Weapon, Darkfire, Greater Shocking Varnish and Deafening Clang!