Is the pain within
the shoulder blade outer edge (rotator cuff muscles) or outside
it in the surrounding muscles that position and stabilize the shoulder blade?
This Todd Ellenbecker video describes in detail the shoulder anatomy, function, rotator cuff muscles and the strain placed on them by the service motion. He says that the small rotator cuff muscles must stop the rapid axial rotation of the arm and racket and that strains them. Also, starting at minute 8, he describes the serving shoulder posture to reduce the risk of impingement injury. Many servers risk injury by not using the positioning that he describes.
See also this thread and especially Charliefedererer reply #11.
Take a video of your motion from behind to evaluate the service motion. 30 fps with a fast shutter is useful but repeat the serve videos to catch the racket in various positions (since the serve is fast you must assume that the serve is reproducible with 30 & 60 fps), 60p fps with fast shutter works better, high speed video with fast shutter catches it all. Shoot in direct sunlight so that the AUTO exposure will pick a faster shutter speed.
Injury can result from shoulder posture as pointed out to me by my physical therapist. He said that injured baseball pitchers often need correction of their shoulder blade posture. Search shoulder posture.