What can I use to clean my black puppies coat before the big show? All advice welcomed.

I usually take my Labs swimming. Works every time.

Wet them, then massage in some white vinegar let sit, for a few minutes, and rinse. You'll be hungry for anything with Itailian dressing on when your done but the pups will will have a gentle cleansing that doesn't strip the coat oils.

Murphy's Oil Soap. Please clean them up. There is nothing worse than going over a cute stinky, grimy puppy or dog. MOS will not strip the oil or soften their coat.

Basically I just wash them with water, followed by more water, and then some more water, until the drying towels are still clean when you dry them off.
If I use vinegar, I prefer to use Apple Cider vinegar. Makes then smell a bit sweeter!

Radio.

Brown Listerine. It gets the dirt off, they smell fresh but it doesn't soften the coat. Vinegar works also but doesn't smell as nice.