Ford Mondeo black smoke

I got my ford mondeo in february and when we bought it the guy we got it from said that the clutch was slipping and needed changing. That wasn't the only problem, the gear stick went potty and we had to get that done also and not only that it blows out black smoke when you rev the engine. We have had the clutch and the gears sorted but still have the problem with the black smoke. We had it serviced at a reputable garage; they checked it over on the computer and checked the fuel injection side and they seem to think the catalitic (sorry don't know how to spell that word) converter could be blocked. They also put a fuel additive in the tank. I am having massive loss in power in my car and lots of black smoke coming from the exhaust. When on high revs for a while it seems to have a fuel loss and lack of power like no fuel is getting through. I hope you can give me some advice on this as it is driving me nuts..