You can't have a single grapevine that produces both white and black grapes. what you have is a black grape variety and you had picked them last year before they had ripened. They start off green then turn black when they are fully ripe. Mine have just started to turn black and should be ready to pick about the end of September, beginning of October. I'm surprised you didn't find them bitter and inedible.

If the grapes you had last year were 'normal', that is sweet and not inedible, the vine might have mutated back again - white grapes have two mutated genes which prevent the production of anthocyanin. Doesn't seem likely that two genes would have mutated though, but its not impossible. Whatever the cause, assuming the grapes are actually grapes (taste one to see) and not some kind of rogue fruit growing off the rootstock because the original, grafted vine has died, there's no reason why you shouldn't eat them. If they were coming off a rootstock, they won't taste nice at all...and should be smaller.

It's very unlikely that the vine has mutated Bamboo. If, in the unlikely event this has occurred, then the chances are that a red grape would mutate to a white grape and not the other way round. As Joan said, she had white grapes last year. If the grapes are growing from a branch off the rootstock, i.e. below the graft, then I'm sure that this would have been noticed, unless she cut it right back below the graft. Also, most grape vines that are sold in the UK for gardens aren't grafted but are grown from cuttings as they grow so prolific and so it's more commercially viable. I think we need to find out what has gone on with the vine. When I have some time I will PM Joan.

Myron, I've simply offered a response based on the available info in Joan's question. It may, or may not, be of interest to her, I don't know.

I understand what you're saying Bamboo, I was just trying to rule a few things out and hopefully sort it out. But as you said, with the lack of available info it's difficult to know what the cause might be.