I have blue rtv here, will that work for a thermostat housing?

what do most people use?

Blue will work - I use it for the outlet from the cylinder head.

blue is what I used. :)

Blue is the one your "supposed" to use for coolant area sealing. I honestly don't know what the difference bewteen the different RTV is whether its just the color or they have different properties.

Blue works great. On my crx beater one of the two bolts broke off of its thermostat housing, but with gobs of blue it hasn't ever leaked. JB weld also makes a fine "gasket" for thermostat housings. Don't ask.