When you wipe it off, is it solid or does it smudge/break apart? It could be a kind of mildew - I think it's called sooty mildew or black soot or something - that grows on the nectar secretions of plants. If so, it's pretty much harmless; just means that your plants are happy and not getting rained on enough.
o//~ Livin' like a bug ain't easy / My old clothes don't seem to fit me /
I got little tiny bug feet / I don't really know what bugs eat /
Don't want no one steppin' on me / Now I'm sympathizin' with fleas /
Livin' like a bug ain't easy / Livin' like a bug ain't easy... o//~
Thabks for the reply, it does come off completely as one piece but can smuge a bit too, most of my plants seem happy but you never know effects might not show yet if it is harmful. I have never seen or had this before and just need to put my mind at rest as to what it is.