Seen this allot - the issue/challenge is if it was bluish originally
Normally the easiest one to notice is that upper corner that faces the driver and passenger window
Should remember to take a seat apart to see if the stitching in side (not exposed) is the blueish or is black
Not that its related at all, there are a number of things related to paint that look black on Mustangs that go on blueish but build to a solid black
Two that stand out are the 69-70 gauge surround paint and the hood black out. Looks really odd on a Coral hood when you do the first coat. By the 3rd pass it's built up to a black