Try "Dawn" detergent. If you have oil spatter, as I have from a leaky head gasket, it removes it quite easily. I just got a hot wet rag, a few drops of Dawn, scrubbed and rinsed.
You did not say where the black spots were. Are they on the sides of the seat & bags or on the top? If the spots are on the top you might be seeing bird droppings , mold or pitch. I like to try a non-petroleum product on these things first, like vinegar. Peanut butter will remove pitch but the peanut oil might stain the leather bags and you have to wash with a detergent to remove the p'nut butter afterwards. Some folks have used WD-40 (fish oil base) but again the bags might stain (try it on the underside of the bag to test).
I don't want to get into catch can issues, that a whole other, much discussed to death, topic. Leave it with the notion that many (most) of us here, have deleted the can setup for a simpler tube and duckbill variation.
-Heloise (aka Jesse in P.A.,WA)