As a smart 4th Grader in 1979, I avoided both this and the Star Trek movie in theaters. The reviews (and I did read the local movie reviews) had said ST:TMP wasn't real trek, which was enough for me. Black Hole probably looked boring, and I was old enough for the Disney name to turn me off. At the time I was into the cartoon Star Blazers (the anime Space Battleship Yamato), with tons of space combat. The sad thing is, I later avoided Dragonslayer because of the Disney name as well, and that's great. Ah, what Disney could have been if their attempts at more serious genre movies and animation (Black Cauldron) had done well...
While this was Disney's first PG film, that's only because of when it was released. When Disney released Treasure Island on video, they had to edit out some of the violence to keep its G rating. Planet of the Apes also had a G rating.