There has been a few threads about this, but I would say:
matt finish.
But the dye does not penetrate all the way through, rubber is coloured before it is formed and because of this it cannot be altered.. My understanding is that it will absorb some of the colour but if you cut it or rub one area for long enough the original colour will come through.
Test it with a rubber brake block - wait for it to dry and then sand away - see what happens.