The engine is looking fabulous!
A wash can certainly help, although I'm not sure I'd go with black as it may stand out a bit too much. Perhaps a brown or grey, or a mixture of the two. Personally I like to use artist oil paints in the appropriate colours, mixed with turps. Runs through the cracks great, and can generally be lightly wiped off the build and won't leave residue or mess with the paint (but be careful, too much stuffing about and it will mark your paintwork).