The black jellyfish is blood-red in color and packs a painful sting. Giant Stinging Black Jellyfish on the Rise: At first glance they can look like a big black trash can lid. But they are gelatinous, float in water and are a disturbing blood-red in color. Black jellyfish, once a very rare sight in waters around San Diego, are now appearing more frequently. "In the last decade or so, we've seen them about five times in our local waters, whereas in the 100 years before that they were seen about five times," said Wendy Spaulding of the Living Coast Discovery Center in Chula Vista, Calif. Spaulding speculates that warming waters could be triggering bigger plankton blooms, which then feed the jellyfish. The jellyfish start out small, but can get as big as trash lids. And they pack a painful sting. "It's not fatal, but it hurts," Spaulding said. So if you spot a giant black blob floating your way, best to swim clear.