Insufficient bleaching of a C41 negative, e.g. due to exhausted or improperly oxygenated bleach, will cause the film base to be unusually dark. There will also be residual silver left in the film, which forms a monochrome image overlaying the colour image, which reduces saturation and increases contrast.
What you have is an accidental partial bleach bypass.
To cure the problem, redo the process starting from the bleach step using fresh bleach and fixer. Don't forget the stabiliser. The negatives should come back to normal after re-bleaching and fixing; if not then there are other problems.
Back to Analogue Photography and Film FAQ