No, eBay will not place a black mark on your account for accepting a cancellation.
I would also strongly advise, accepting it in this case. There's no point sending an item which clearly will not be paid for, so you'll find yourself in a longer wait to cancel for item not paid.
Accept the cancellation citing Buyer Requests to cancel.
This full refunds your buyer, and your seller performance will not be affected.
Block them too, from returning and messing you about again, here:
https://www.ebay.co.uk/bmgt/BuyerBlock?
@merchmarine