bit of black soot on tail pipe why?

corsa 1.2 53 reg we had a fuel problem before found out it was knock sensor any its all ok now but the black soot is not as bad as before it wont go away oh yes when i done head gasket the egr body was blocked up with black soot cleaned all out but we havnt got a egr valve on ours only body just done a service as well also used fuel cleaner once it helped alot thanks