Thankfully I don't intend on just handing my car back and walking away.
Its at a value where it roughly equals what is owed on it.
I had a colleague at work many years ago so things might have changed, that handed a car back as he was over half way through paying the finance. He followed everything correctly that was in his T&C's and they managed to black list him. He had a terrible time getting it sorted. So that route is definitely out.
They legally cannot black list you or otherwise negatively affect your credit rating. It is added to your credit file as per any other credit that is in progress/settled/terminated.
It may affect your ability to get finance on a car from the same company in the short term, but will not and cannot affect finance on any other item like mortgages or whatever.
BMW were rumoured to be difficult if you handed back and wanted another car on finance but I saw no evidence of this, It made no difference to us either in actual credit rating or in having the full choice of cars and finance companies.
I suspect if other items on a persons credit file point to being a risk that adding a termination is going to make vehicle finance tough but the termination is not the issue there it's the crap credit rating in the first place.