Black locust does have some special chemical properties that would make me not want to use it for this purpose.  I wouldn't use cedar, black walnut, pine or redwood either. 

Doug fir is considered not as bad. Spruce and true fir would be better than Doug fir.

The guy with the Mason bees in Seattle urges people to use reeds. They seem to work better than wood blocks. As we've said on this site, teasel works really well. Mine fill up with bee cocoons much faster than wood blocks, and they can be individually cleaned.

John S
PDX OR