Sema, who is on trial with Gary Bowyer's side, continued to look lively for Blackburn early on, attempting to control a high ball forward from Anton Forrester, but he was unable to get his shot away in time.
Tranmere's first chance of the match fell to defender Liam Hogan, but the full-back sliced his shot well high, while Steven Jennings was denied shortly after as his first-time hit was deflected behind off Scott Wharton.
Lois Maynard could have opened the scoring for the hosts early in the second half as he fired a low shot towards the Blackburn goal, but David Raya made a save at the near post to keep it out, while another shot from the new signing went wide a couple of minutes later.
The Championship side soon started to dominate possession though, with triallists Sema and Jeffrey Sarpong combining, resulting in the latter turning the ball wide at the far post.
Sarpong made amends inside the final 15 minutes, unleashing a swerving effort from outside the box which sent the keeper the wrong way to finally open the scoring.
It did not take the visitors long to double their lead as Connor Mahoney carved out a chance inside the final 10 minutes, cutting inside before rifling the ball into the bottom of the net.
Connor Thompson could have added a third soon after as he collected the ball on the left-hand side of the box before attempting to slice the ball in from a tight angle, just missing the target.