Are there any penetrating sealers for concrete countertops that will darken the color?
There is a method that can solve both of your problems. I’m not sure how to translate it in English but it is called something like "polishing up to a point of black gloss“. This method makes the concrete waterproof and also brings up the color like you wanted. Back in the past this is the way people made their floors waterproof. Basically you polish concrete with something like fine sand stone till your hands start to bleed and then some more. Today you can use machines called "helicopters" which are rotating steel blades and high speed producing same effect.