Well, it probably wouldn't prove that GBC interfered with ABC's body, but it would prove that ABC was bleeding in that area of the car at some point after she visited the hairdressers on the evening of the 19th. It would be pretty condemning circumstantial evidence imo.
But, if it was found that the hair was not recently dyed (or that the blood stain was older), the opposite would be true.
Edit: I recall the hairdresser saying she would do foils though, in which case, only part of her hair was dyed, which would then mean the opposite would not necessarily be true, just more likely.

I'm also not sure whether tests would even be able to tell the difference between hair that was coloured that day or four or five days prior either.