I got DH to knead the colour into 1.5kg of ready to roll and it didn't take too long (would have taken me a lot longer though). Proper Black might be harder to achieve though and I'd probably just buy it instead you can order online
Like others have said, it's possible but I wouldn't recommend it. I forgot to order black icing for a cake a few months ago and needed quite a bit. I used the gel stuff and made some. The actual result was fine but it took a loooong time to get black rather than dark grey, used a huge amount of colouring and was a total PITA!
This will be too late but I have found that leaving the icing overnight helps deepen the colour. I coloured R2R icing black not that long ago. Iced the cake and left it overnight - was deep black the next day.