I'd say to match a weave to Bey's natural hair color, you'd need a 2 or 4 which is deep brown. Her hair isn't black or even off black (1B) and neither is Solange's. Solange dies hers a jet black; when she first cut off her relaxed hair down to pretty much nothing, it was dark brown.
Jules was born with damn near blond hair and it's dark brown now. And Blue's hair has darkened up more to an off black/deep/deep brown, but for a while hers was that same shade of brown that Bey/Solange had as kids:
