In can be a suffix but when a surname ends with the letters in it is nowadays rarely (never ?) understood as a suffix, even if centuries ago it would have meant something.

The suffix in added to a noun means “of or pertaining to, of the nature of”, it shows the relationship to whatever that words represents. The same as the English suffix ine.

A citadin is someone from the city (cité).
An angevin is someone who lives in the town of Angers (or the Anjou area).
Chevalin is an adjective to show the relationship to horses (cheval in French).
Serpentine is a spotted stone that looks like the skin of a snake (serpent in French).
An Augustin is a monk belonging to the order of Saint Augustine.

Giving people surnames started in the Middle Ages in France (and in a lot of European countries I think, the custom is a lot older in China I’ve been told). The surname was given according to the person’s location or job. But nowadays, even when people know that, they have forgotten the origin of their surname, or just do not think about it, it has no significance to them whatsoever.

You say “many French surnames end with the letters IN”, I wonder where you got that idea, “many” would not be the word I would use. Among your examples, Blouin, Potvin, and Jolin are fairly rare, only Morin is relatively frequent.
The most popular French surname: Martin does end with IN though. It was a first name before it was also used as a surname.

I would surmise that someone with the surname Potvin had ancestors that originated from Poitou (a Poitevin nowadays is someone who lives in Poitou).

Jolin might be related to jol (a Nordic winter solstice celebration, it’s called “yule” in modern English and unknown in France).

Morin could have been given to someone coming from a place called Maury (there are several places with that name).

I had no idea about Blouin but I found it on Wikipedia. It is of Germanic origin, it means “soft” (bili-) “friend” (-win). I do not know of a place called Blouin in France, but there seems to be one in Quebec.

Edit : I have found confirmation about Jolin here. And as you see it is a quite a rare name.