Depends if you want the flowers. If you do, prune after flowering. Otherwise when dormant. I did mine a couple of weeks ago but I think it would have been better late autumn.

I always prune mine after leaf fall. It's easier to view the structure, prune back to leaf axles and take out any whippy stems.

I do mine just as the buds start to move in the spring. Even though I cut it back to a couple of feet, it still carries a few flowers.

I've got two and I think they bloom on last year's wood so I prune one hard after flowering, in the hope it will make enough fresh growth before the winter to bloom the next spring. This does mean you lose the berries but something has to give! I prune the other in winter so that has autumn berries but no flowers the next spring. I alternate so that each bush has a chance to bloom. I do not prune them hard every year though, usually after two or three years depending on the size they've reached.
I was advised to have 2 bushes in order to get better berries but the Black Lace has insignificant berries while those of the Black Beauty are much better. This system has worked well for me.

Crafty planing Pf!

Yes, Stera, but it took me a few years to sort out my system, partly because I hate losing flowers and berries. I should add that the Black Lace has beautiful fine-cut foliage, while the Black Beauty's foliage is similar to the common elder but 'black'. It does make a striking bush though and its berries are definitely fatter!

Well they couldn't be measlier than the Black Lace!