...Since then I have seen a number of other manufacturers
selling one tin for both matt black and blackboard use, so I think e
can conclude that the two are pretty much the same.

Hmmm - dunno about that Fred: I have an old tin of blackboard paint, and
whereas ordinary paint would solidify over the course of time (in this
case: about 30 years! :-) ) this paint has separated out, into a
liquid layer of thin black stuff, over what seems to be black concrete.
I'm impressed with it: I'd like to make a blackboard now, just so I
could try it out.

John