DH is mixed African British and he has no idea either. Is it something that doesn't really exist?!
It does exist, but like all cultures is it is like a bit of fog, you can't really say where it starts and ends, patches of it apply to everyone, patches to only a few. One of my friends will ask me 'Am I being black?' and we both get what she means.
For the majority of cultures there is a shared history, for many (not all) black people there is a shared history of parents or grandparents as immigrants (and in Britain it is not uncommon for a black person to have both African and Caribbean grandparents), of them not being able to buy the food they were used to, difficulty getting a landlord to rent from. But these experiences are also common to Irish immigrants to Britain.
There is also the thing that you do not realise you are part of a culture until someone points out it doesn't happen elsewhere or to other groups.
eg a friend who is born and bread in the south went with his father to Yorkshire to visit relatives. He found it strange that a) they were offered tea at every house and b) when his dad agreed not just dad but everyone was given tea.
Now I was brought up in Yorkshire and to me if a group are visiting and I offer tea I am offering it to the group and if someone doesn't want any it is up to them to say so.
Some examples of things that I identify as black culture (please do not be offended, as stated above these do not apply to everyone and maybe they are actually 'British Black culture') are:
Church clothes - beautiful clothes, and for ladies hats, worn for church on Sunday (or Saturday), sometimes this is traditional African Dress (yes I know it could be from many different countries). I once read a book that phrased it as, "the kind of clothes black people wear every Sunday and white people only wear to weddings"
Washing meat before you cook it - although this maybe more a Caribbean thing.
Sugar in plants - the 'fact' that the amount of sugar in a plant is different whether it was grown below or above ground. Again this might be a Caribbean thing.
Hair. Borrowing from Zadie Smith's 'white teeth', black women expect a trip to the hair dressers to hurt. If I have a perm I leave the the hairdressers with curls, a black woman with a perm leaves with straight hair. If I go swimming I m not scared my hair will break off afterwards
The friend with the 'am I being black', the kind of things she does (and I have not noticed it with other people so maybe it is just her) is if we go to a restaurant she will pick up the cutlery and rub it on her napkin to make sure it is clean, I assume it is clean and if it isn't will ask for it to be changed.
Dayshiftdoris the same friend with the tea has also been taken to Lancashire where he was also highly amused at the thought of a pub quiz with a 'pie and pea supper' and could not believe his ears when someone in a chip shop asked for a 'potato pie butty' and could not believe his eyes when it was made and handed over without question.
My parents don't really trust me because I like chips and gravy - well they should not have moved me across the pennines at an impressionable age.