I don't have a black ice radiator, but that sounds about right. If you're air temp is 75F, I'd say 77F is a good water temp. With just a radiator, you can't make water temps get lower than air temps. It's not physically possible.
I've never waterooled, but I'm somewhat familiar it. I hate to sound remedial, but if you want cooler temps, you'll have to either pass cooler air through the radiator, or use something in addition to the radiator to cool the water. (i.e. fridge....)
sorry I couldn't be more helpful.