In May last year, the blue-and-yellow national team striker scored the only goal in the Champions League final to give Arsenal the title against Barcelona.
It could be a second consecutive title for Arsenal - thanks to the 3-1 victory at home last week. Then, Blackstenius scored the 1-0 goal.
At Stamford Bridge, Sjoeke Nüsken put Chelsea 1-0 up in extra time, but the home team didn't get any closer than that in the return.
Blackstenius headed the ball into the goal in the 80th minute - played in by substitute Smilla Holmberg - but was judged to have been offside in an earlier situation after a video review.
The semi-final awaits the winner of Thursday's match between Lyon and Wolfsburg (0 - 1).
