sport trac shaking some,rpms go up and down a little, black smoke from exhaust?

[mdlott]

My husbands sport trac has recently started shaking a little and the rpms will move up and down some. Also black smoke will come out of the exhaust pipe. The black smoke has only happened once. The truck is a 2003 and has about 89,000 miles on it. I am no mechanic but it seems to me this could have something to do with fuel something or other? Please help! He is taking it to the shop today, but he and I are different in a way that if they said he needed a new engine he would say ok. Any advice would be great!