I was talking with a friend whom asked how to get blue backgrounds and how to get black, but when she wanted them not just how they happened to turn out that day.. I told her that sometimes just tilling the camera up, down or level can give those results, she looked at me in disbelief, so I said I would show her, these are the 2 images:
This taken with the 105MM ISO100 1/200th and f/16 strobes (DS125's X2) at 1/4 power AND TILTING UP and waiting for the sun to come out for that shot. 40ft deep

This is the same Blenny taken seconds after same ISO100 1/200th and f/16 strobes (DS125's X2) at 1/4 power BUT HOLDING LEVEL, and I moved just a little to get a dark subject (coral head) behind oh and the sun went behind the clouds at that time. 40ft deep

I have to say thanks to Alex for understanding how to get these colours when you want them. Some times the subject is better with a black background than a blue and vise versa, it's nice to know how to do it in camera rather than later in PS.