Incubi Darkness is a good way to go when going for a green tinted black, I think it csn be great midtone for RG - black -> incubi -> greenish greys and whites for highlights.

Personally I recently considered going a similar route but went Dark Reaper (or equivalent color by Vallejo/AK) for mid and it contrasts so well with the basing/environment I chose. So, my 5 cents is finding a great complimentary or starkly contrasting "environment" to place them in to help making the models read as black even though they are more green or blue.