If you have FUE to the scar and want to keep your hair short it may look WORSE since the transplanted hairs may not exactly match the surrounding native hairs. It would work if you want to grow your hair long over the scar.
Scalp MicroPigmentation is likely the best first approach. You would still need to consult with a doctor to make sure this may be an option. See VIDEO
Blood supply is not the issue.
Sometimes density alone over the scar area may not be enough to create the ideal cosmetic aspect desired. This is due to the color contrast between the skin and the scar tissue. A better solution would be a combination of FUE and micro-pigmentation. With exceptionally large scars and elastic skin found in some cases, and to avoid the wasting of valuable grafts, I would suggest scar revision with an experienced doctor prior to FUE and pigmentation.
This works ok, but as you point out the density as a real problem in just a single session. Scalp micropigmentation is a much better solution as it always hides the scars nicely. Look at the scar section on the website below to see what I am talking about.
FUE is a viable option but you need to be examined by an expert to see if it is an option for you. Scar removal with realignment of hair follicles is also an option but only by an expert. I perform both for scar revisions.
The best way to maximize the results of an FUE procedure to a donor scar is to use PRP (Platelet Rich Plasma) at the time of the procedure. PRP contains natural components that assist in the healing process and the better growth of the transplanted hair.