How do I remove black (someone said it is mold) from cedar siding?

Look under Roofing on the Internet. Bet there are lots of suggestions there. If it is dangerous black mold (ask a professional) because handling it can make you sick. Leave it to a professional.

Here's a vido that might help...

Black mold is not something you should deal with on your own. When you brush it or try to wash it, it becomes airborne and can make you very seriously sick. Best thing is to find local business that specializes in mold abatement. It can be tested and if it is black mold it can ne removed. If it's not there are cleaners available tgat you can use with your garden hose to wash it off. Please do the safe thing.

Hello. Black mold is very toxic to breathe in and it spores can cause serious illness and even death. If it is truly black mold, contact a professional to remove it. They have the appropriate tools and suit to wear during removal. Good luck!

Probably it's a combination of not being sealed, weathering and mildew. Go to a paint department and speak to them or go to a home improvement store.