I add molasses to it, but it never gives the cake the dark colour I want. I read a recipe with coffee and cocoa in it, but wouldn't it change the taste of the cake?

organic black strap molasses is extra dark — what about using coconut sugar which is already brown to replace white granulated if indeed you even use the white —

i’m racking my brain to think of something else like a fruit that you macerate and put in the food processor to help darken things — nice dark raisins would help — macerated and ground up — without affecting the flavor adversely —

actually it’s time that can help increase the darkness — sometimes people steep  fruit in sugar and booze and tend it over time that gets black  — it’s a caribbean thing —

I think the coffee will help darken and enhance flavor — if you use black cocoa powder — like they use for Oreo cookies you could probably get away with a tablespoon or two before the chocolate flavor takes over too much — that should really help — amazon  has it — it’s black like soot — 

I always soak my fruit cakes in a 50/50 mixture of grape juice and rum, which makes it turn very dark.  I wrap the cakes in several thicknesses of paper towels or cheese cloth, then a couple of layers of aluminum foil.  Before sealing the foil, I liberally soak the towels and add more of the mixture whenever it has all been absorbed.

Over here in the UK we love a dark fruit cake - we use black treacle and I believe molasses is your equivalent. We also use dark brown sugar. The  Delia Smith Christmas cake recipe is great