I wish I still had the pic of my birthday cake 2 years ago. It was a coffin
My mother bought black pre made icing roses from a baking supplies shop. Basically she made an apple cake and we cut 2 corners off, and a little of the side to make a coffin shape when it was cooled.
Black icing is made with black food colouring. It
does exist, I have some in the cupboard. I made some black coloured, lemon flavoured Bauhaus cookies not too long ago.
Try a specialist baking store for food colouring and so on.. You may have to look online if there aren't any actual stores near you.
You can make up some thick but runny white icing, put it into a very small UNUSED syringe, and draw your designs on like that ( I do it this way and it works fine ) Or you could make some stiff icing, ( use lots of icing sugar ) so that it is the consistency of modelling clay, colour as you wish and make gravestones, roses, ect like that.