Have you tried it? It might be fine. I can't ever remember my blix being jet black. It has been a few months since I did RA4 but ifI recall correctly it was a maroonish/burgundy-brown on mixing and went more brownish after use.

The jet black bit when freshly mixed worries me more than the muddy light brown. I cannot see any reason why the blix should have gone off given what you have said.

pentaxuser