Blue groove is a dark, polished lane worn into a dry dirt track where laps have packed and rubbered the surface. Dirt oval racers usually mean the grippiest line; motocross and RC racers usually mean a slick one.
Ask which discipline first. On a dirt oval, a blue groove is usually rubber — as the track dries, cars lay a dark strip down the preferred lane, and that strip can be the fastest real estate on the property, which is why some racers hunt it. Other dirt racers say “blue groove” as a complaint, because a rubbered-in single-lane track takes the passing out of the race. In motocross and in RC, the same two words mean hardpacked, polished clay with no loose material left on it and a tire that skates. There, blue groove is never a compliment. It is pit usage, not published terminology: NASCAR’s dirt glossary defines the cushion, tacky and slicking off but carries no entry for it, and Wikipedia’s glossary only the word groove.
Blue groove means grip to a dirt oval racer and no grip to a motocross or RC racer, so carrying setup advice across disciplines on that one word points you exactly the wrong direction.