If an item and or attack power in 4e gives a creature both the blinded and deafened status effects and the creature has no alternative senses (blind sight, tremor sight) is it reasonable to treat all other creatures as hidden from the blinded and deafened creature?
The blinded condition doesn't do that.
People can hide anywhere from the blinded creature (since they have total concealment) but they're not automatically hidden. If they do hide, there's a -10 penalty to the Perception check to find them.
The only effect of the deafened condition is an additional -10 penalty to Perception (it is not total deafness) (RC230).
If the enemies don't hide, only a -5 to the blinded character's attack roll (for total concealment) applies.
For further reference, the Rules of Hidden Club might be useful.