> I have seen the description "block plane" used on transitional planes in
antique shops presumably because of the body/block being wood.

I've seen that as well.  To some antique dealers, and some eBay sellers,
any plane made from a "block of wood" is a block plane. 

> The Stanley 25:
http://www.supertool.com/StanleyBG/stan4.htm
Is the only transitional with a bevel up, so is the one that I have
always thought of as a block plane, and Mr. Leach agrees.

I'd have to agree with Mr. Leach.  Bevel up, and (relatively) low angle
seem to be the defining characteristics. And relatively short.  My D.R.
Barton cooper's long jointer's blade is set at approximately 30 degrees,
but is not short (g), and is set bevel down for all that it is made from
a block of wood. 

Mike in Woodland