Pruning Sambucus nigr Black Beauty

I don't think he means as they grow~this is from BBC Gardeners World

One of a number of ornamental varieties bred from our native elder, 'Black Lace' makes a striking plant for the back of the border. It has very finely cut, almost black foliage, which is the perfect foil to the pink-flushed blooms. It will grow almost anywhere, including difficult conditions such as waterlogged or very chalky ground. In autumn, leaves turn rich red. To produce the best coloured leaves, prune plants back to ground level every year in early spring.

I don't think he has told you correctly either.
EVERY piece of that plant starts off green.
As each piece matures (over the days/weeks) it turns into the dark/black colour.

If you trimmed each green part off it wouldn't grow, as Arelene's just mentioned .... cutting it hard back will keep it under control (because they grow very very fast) however .... i just leave mine alone and they're both beautiful plants (i have 2, they'll be in my photos).

Mine is not big enough to cut hard back - I trim it in the spring to keep it to shape, and so that I can still enjoy the pink flowers.

Louise is right- you've had wrong information...don't cut all the new growth off!

~mine is a small one too~all the new green shoots have turned black already~I am not thinking of cutting mine back either as it is just starting to get a bit of shape!

I grow 'Black Lace' and new shoots are always green before turning black. If you grow them in deep shade, the leaves also turn green.
Elders like this are so easy from cuttings, either softwood or hardwood, so it's easy to get lots of new plants to make a hedge or a clump of these.
The flowers of Black Lace are superb, a white flower with pink centres.
Just about my favourite shrub.