If something just darted across your block wall and froze, it's probably a Western Fence Lizard. They're small (3–8 inches counting tail), brown or gray with a pattern, and the males flash bright blue patches on their bellies — that's where the nickname "blue-belly" comes from. They love sunny fences, rock walls, and the tops of cinder blocks. They eat ants, beetles, and spiders.
Here's the cool science: Western Fence Lizards carry a protein in their blood that kills the bacteria that causes Lyme disease. When a tick that's carrying Lyme bites a fence lizard, the lizard's blood cleans the tick out. So fewer Lyme-infected ticks means lower Lyme rates in California compared to states without these lizards. A backyard friend with a built-in disease-fighting superpower.
The other one you might see is the Southern Alligator Lizard — longer, snake-like body, short legs, and a tail that's often longer than its body. They look intimidating because they're bigger and they will absolutely bite if you grab them, but they're not venomous and they're not aggressive. We have one as a pet and he's great. Alligator lizards eat slugs, crickets, and even small mice — they're tougher than they look.
Both species are protected to some degree in California and shouldn't be collected from the wild. If you find one in your house, the kind move is to gently usher it back outside. It was probably hunting something that you also didn't want in your house.