Id say youve been pretty lucky with that particular pre-blend roast Secksy...... From my experience with the Dominican bean it is a very hard and tough little bugger, even tougher than the Yirg and definitely much more so than the PGE. Would explain the wide variation in roast colour.
If you want to give this blend a retry, Id roast the Dominican separately to the others which would probably be ok together and then blend the lot post-roast. All in all though, this blend would probably work pretty well together but with maybe 20-25% Yirg and 15-20% Dominican. The Dominican is very nice on its own though and worth trying as an SO. All the best,