The idea of known quantity v unknown quantity implies that by going for the former you remove an element of risk. But I think managerial appointments are always gambles, IMO. You get very few managers who are successful everywhere they go. You can never guarantee success. You just need to find the right man for the role, and at each club that role totally varies - how much money they'll have, whether there are young players coming through to support the squad, what the expectations are, how much squad turnover is needed, the average age of the squad, how much control he'll have over the team + transfers. For that reason, if you think Blessin is the right guy, you go for him. You don't just go for someone with experience. I think a lot of other people have talked about why Blessin ticks a lot of boxes. For me, having the managerial experience, even if it's only a season, is more important than having experience in the UK. He can research the league, and he speaks good English.
It might be a gamble, but managerial appointments always are.