Why does the Dragon Master carry a Black Monarch's shield?
Well I think I can answer it from TLC POV:
1. In 1995 when this set was released, the Black Knights theme were unavailable anymore. I think TLC weren't sure if to kill it completely or to still keep a bit of the flame sparkle a bit longer - maybe even a come back which not came. You can say the same about the 6079 set were there are still 2 DM knights even though the DM theme was killed in 1996. And even more they have the old Black Knight's Shield with them. So maybe they were just fading them off slowly.
2. Or simply many shields that were printed already left to get rid of.
(Come to think of it maybe the entire Dark Forest theme was TLC plot to get rid off all the Forestmen/Wolfpack leftovers and that's why they were so very difficult to find)
On the other hand from the minifigs POV I guess:
With the gone of the Black Knight theme, the DM knights could use their shields which looks almost the same as their own.
(or in other words you can say that the DM simply defeted the Black Knights and theirfore could use their shields, hehe)