black really is one the worst (if not the worst) colours for showing sctratches, looks nice when all shiny and polished but so easily marked.
As said depends on how deep it is, if it is that bad the only way is to paint the complete panel,as a bodyshop we are doing this all day long on sales cars, if there was a viable alternative the sales guys would use it.
Colouring it in with black felt pen wont get rid of it but can take your eye off it and it wont be so obvious.