Hello!
Having black oil comming from the engine is not good and indicates that you have set the engine to lean!!
Ofcourse you can go back to the 5% nitro blend.
At my club here in Sweden most people (130) blend their own fuel, and so do most people in all the big clubs around here in Stockholm (Capital of Sweden).
Glow fuel is just Castor oil (or synthetic or a mix of the two), metanol and nitro (nitro is not necessary).
Over here in our club, and the rest of Sweden it is very common among fliers to use 10%-15%all synthetic oil and 5%-10%nitro.
Regards!
Jan K
Sweden