Woof Danny Boy:
Posting from the airport in Mexico City, allow me to give LG Russ a helping paw.
In traditional FMA consistent use of terminology between various systems is prohibited-- so traditionalists that we are, we use the term umbrella completely differently from anyone else.
As you correctly observed, in many systems the umbrella IS similar in appearance to a roof block but differs in meeting the force less directly and instead guiding it around the head (knowledgeable people please forgive the crude description). In other systems it picks up the strike almost with the back edge and, again, guides it around the head.
In DBMA a roof block is a palm/tip down block- most commonly used by matching-handed fighters (e.g. righty vs. righty) to defend a caveman strike (#1 in most systems).
An umbrella is a palm up/tip down MERGE/parry against backhand strikes by the opponent-- often flowing into backhands of its own. Top Dog is a master of this motion, and Lonely Dog does it quite well too. Typically it is done in Weapon-Largo range. At closer ranges (medio, corto) it becomes a secondary block, with the principal block being accomplished by the checking hand.
So why the different name for us? It goes back to when I was helping Top Dog prepare his presentation (1992-93?) for our first series. He lacked a term for the move this is what he was able to remember