No, you were right with what you said originally. Yes, a 15A rated cable will pass 15A whether at 12 or 230v, but it will need to be larger to keep within sensible volt drop limits @ 12v. Probably not applicable when such short distances are involved as on a motorbike, but hey ho.
Back to the OP, what is the blue fuse trying to indicate? Is this negative?
Red is 10a and blue is 15a