A black painted leading edge? Usually that's from someone removing the boots and then painting it so it looks the same...what other shortcuts have been taken?
Check to see the condition of it. A lot of people use automotive paint which is not good enough for something as important as the leading edge.
I remember reading somewhere that it encourages separation and raises the stall speed a few knots.
Performance wise, worst case scenario is that it will cost you a knot or two. That wont be the main source of your problems. Just a contributing factor.