Sometimes you'll see a channel which has a darkgray or a black icon in front of it.
When you click on it will also show 'trap' or 'blocked'.
Those are channels that were reported by a significant amount of other people as being trap channels or are blocked for whatever reason.
Those cannot be joined.
Update 26/06/2021: this has now been tweaked a bit in v3.0.0.9. Trap channels can be joined by setting the "Allow trap channels" in "Settings->Networks"