Is this a variety of banana slug (Ariolimax columbianus)? (California)
Banana slugs are comprised of three species: 


Ariolimax californicus 
Ariolimax columbianus
Ariolimax dolichophallus


Ariolimax columbianus, or the Pacific banana slug is known to have black spots, sometimes so large as to cover their entire body (Source). This is certainly the genus of the slug you've shared.  

At times, you could be seeing a Black slug which is well-established in Washington and Oregon. I only include this information as I have noticed this species occasionally in Southern British Columbia and wondered if it was the same species as the Banana slug. It isn't. The picture you've attached is not a Black slug (note the pronounced frills on the back of the Black slug specimen)