Make sure the loop is AFTER the pre amp stage and is in parallel . I tried this with 2 different guitar amps and found that the efx's loops are not set up like a mixing board loop. Both the ones I used if you looked at the circuit diagram took the signal directly at the input BEFORE the pre amp. ( series) In other word, just the same as plugging the guitar directly into it.
Those rack efxs are designed to go in a efxs loop of a mixer so are looking for line level. So with mine ( Roland ) it was to hot a signal coming from the guitar and sucked my tone right out the window.
If your lucky the blackstar has a real parallel efxs loop.
otherwise your stuck with adding the efx via a mike or line out to the mixer/interface.