What kind of black spider has a white or yellow heart on its back?

The spider you are describing is likely the "black widow" spider, specifically the Western black widow (Latrodectus hesperus) or the Southern black widow (Latrodectus mactans). These Spiders are known for their glossy black bodies and distinctive hourglass marking, which can be yellow or white, located on their abdomens. While they are venomous, they are generally not aggressive and will only bite in self-defense. Caution is advised if you encounter them in the wild.