I was told blue/black as well, so it's the one I tend to use. Funnily enough, although it is supposedly darker to my eye it is also 'softer' somehow, more attractive than just black.
I did a friend the other day who is dark blonde and usually wears brown mascara... used a mix of blue/black and brown but used more blue/black than I meant to - oops! However, she loved it! Although her lashes are now dark they're less harsh somehow than just slapping on the old black mascara.
I only actually have blue/black and brown in my kit because they seem to cover all eventualities. The brown is still enough to tint my mum's eyebrows which are thin now, but naturally very dark, so I didn't feel the need to buy a black at all.
By the way, were you taught by salon systems? Because we were, but advised to buy their Combinal tint rather than the standard one because it's meant to be better quality - don't know if that info might be useful for peeps!
xx