Sanding out weave of fiberglass

Experimentation would be the best idea.

 

Your first step would be to determine what the blue is. Pigment in the resin ? House paint ? I would wipe it with acetone and see what comes off, perhaps even a wipe with styrene monomer.
If the glass is thin you will have to use a chemical etch, if its thick you may be able to sand/grind it.
If the blue is bonded on such that it is impossible to remove then probably abrade with a wire brush or similar, wipe with thinners and apply an epoxy high build.
Perfection is the enemy of good (enough) so you need to determine where the line is, sand it back to clean glass and lay on a resin/veil coat followed by epoxy high build and 2 pack polyurethane ?
Or wipe it with acetone and spray it with waxed gelcoat ?

 

Just sand blasting it all off is probably the easiest quickest way to make sure you have a good substrate. There will be plenty of elbow grease expended fairing that mess as it is.