Catalytic converters were taken off my XJ8 what would cause it to blow blue smoke out the tailpipe

 

Blue smoke is oil burning. The end. Valve seals or rings are shot.

 

+1 to wfooshee's comment.
From my limited experience, bad rings generally cause oil smoke under load (acceleration), whereas valve stem seals will show up more when engine is working on over-run.
So a quick test is to accelerate hard to see if you are leaving a persistent cloud behind you, then take your foot of the accelerator and let the engine act as a brake for 20 -30 seconds, then just a light touch on the accelerator to see if you then produce a noticeable brief plume.
A brief downhill run is good for an over-run test.

Occasionally on start up I have noticed a big puff of blue (perhaps one start every 30-50 starts) which I think is some condensate it the PCV lines that accumulates and then get drawn into the intake in a burst creating a brief but very noticeable cloud.
Other than that occasional oddity, the car runs clean and is not using much oil.