The paperwork for Blackman's move to Home Park on a six-month contract has now been finalised.
The left-back, who was a free agent after being released by SPL champions Celtic this summer, played the first half of the weekend's friendly against Yeovil, having trained with his new team-mates earlier that week.
After the match, Sheridan said: "I think he's a good player; he'll give us a lot.
"I thought he did okay. He hasn't trained much and he's new to the players but I think you could see his ability on the ball.
"He's got to get used to how we want to play and where we want to send players when they're in deep areas because we're all different. Different managers want different things.
"I'm sure in another week he'll fit in with team shape and how we want to play and take it on board because he's only been here for a day or two.
"I've seen him play a few times and I think he's an excellent footballer. That's why I've brought him here."