a Hi Flow Cat isnt gonna throw a CEL...my advice would be to read the code to see what it is for sure...if you are blowing blk smoke then you are more than likely running rich.
this smoke combind with a CEL if I had to guess you may need to replace O2 sensor...if car has been running rich for sometime carbon may have built up and failed the sensor....Most manufacturers recommend replacing O2 sensors at about 50k miles...
so it cant hurt if youve never done it....

OEM part is expensive as all hell...most ppl use universal 4 wire BOSCH O2 sensors from the Auto parts store but you will have to use your factory plug so cut it off leaving you some slack and solder/shrink wrap it to the new sensor.


another suggestion would be to remove your EGR valve and blow it out...look for extreme carbon build up there also if running rich and blowing drk smoke....
both EGR and O2 are the only things I can think of that would produce both a CEL and smoke