My first inclination is to use bleach & water to 'clean' the siding. The 'black' you refer to may be mold / mildew. I don't know what bleach will do to cedar siding.
You're not 'bugging' me! You can still go to your local SW Retailer and ask there since you have to go there, anyway, for the stain you need. (Who knows when Steve (or even I or anyone else for that matter), will 'respond' w/an answer.) The Retailer will, obviously, have an instant-answer.