A brand of cider made by Matthew Clark. The draught version is 5% alcohol, whereas the takehome stuff in cans and bottles is 6%. In terms of alcohol content, this almost makes up for the fact that cans typically contain only 440ml compared to a British pint of 568ml.
Having said that, cans of Dry Blackthorn seems to be on almost perpetual special offer, such that they contain a full 568ml pint.
Dry in taste, obviously (but also available in a sweet version), Dry Blackthorn has quite a clean taste, but tends to have a slightly metallic tang about it. I would expect most people to prefer the broadly similar Strongbow, but in the UK at least, virtually all pubs only serve one or the other, so there's rarely a decision to make.