Very late but Ill post since I just went thought the same thing with my st. marc limestone fountain.

I first applied a sealer on a test patch and it gave me a blotchy area that I was very unhappy with. After a little research I then found out the the key was to make sure the surface was very well sanded for the stone to take the sealer evenly. Did that and was still not completely satisfied with the tint and the consistency of the application. more research then uncovered a colour enhancer product that does exactly what you are looking for, creates a wet look and brings out the grain of the stone. Did this , 7 or so quick and easy coats, and now my greyey blue limestone fountain is darker with subtle brownish grain to it and appears more like granite than limestone. Now I have the perfect contrast with the limestone patio slabs and the limited splashing that was standing out on the fountain wall is completely unnoticeable against the now darker surface.

So it is a stone colour enhancer product that you need. available at the top end fine interior stone suppliers.

there is a wax too that can be applied but I am less familiar.