Mold is very bad in hummingbird feeders. Remember to change the nectar in your feeder every 2-4 days, regardless of whether the nectar has been used. In hot, humid weather you might even have to change the nectar every other day.
I find the more often I change the nectar the easier it is to maintain the feeder. You won't be battling any black mold and you'll have a lot more hummingbirds and orioles.
To clean the hard to reach places you can use a pipe cleaner or we have special little brushes for cleaning feeders at Wild Birds Unlimited.
Now that we are in the hot and humid weather, you should soak the hummingbird feeder for about 5 minutes in a Scoot, active enzyme cleaner once a week. Or use a one part vinegar to nine parts water solution to clean the feeders. Then rinse your feeders thoroughly.
Nectar (sugar water) recipe
1/4 cup white granulated sugar
1 cup water