Firstly, Squall's hair is brown with very light brown, possibly sandy blonde highlights. NOT black.

Nextly, high intensity light on low light film (and other such low light techs) can cause brown hair and other shades that might appear 'dark' under normal light to become much brighter. The fact that Squall's hair has very light highlights VISIBLE elsewhen in this scene makes your 'radiography' less and less plausible and more and more illogical by the second.