Bloo Kid is a throwback to the older, 8-to-16-bit art style of the days of gaming yore. As the aforementioned Bloo Kid, you traverse levels that only take up one screen, killing enemies until they stop spawning. There is no scrolling involved, which I suppose is meant to be part of its "old school" design. Health is handled by Zelda-style hearts.
To add incentive along the way (and to unlock further levels) you are tasked to hunt for stars: one for killing all enemies, one for surviving without getting hit, and another that arbitrarily shows up before the level ends.