Hi Camborne, they are gorgeous aren't they, basically they are elderberry trees in black. So you can keep it in a pot , but watering will be a chore, you can take out the leader shoots and force it out wards rather than upwards. Or I recon because the new growth is quite pliable that you could wall train to reduce it's footprint.
Saying this I've had one in a pot for three years and it flowers but it's not happy unless well watered. Good luck with them x sun loving