I was asked to paint an exterior of a cinder block chimney and need to find out what type of exterior coating will allow water vapor pass through the block so that it doesn't ruin it from the inside out.
Any "breathable" exterior coating will work. The traditional one is limewash, others are silicate paint, or silane-siloxane sealer as mentioned by @LeeSam.
You want to use a silane-siloxane sealer. You need a sealer that is penetrating (will provide better uv protection) to keep water out, yet still be breathable.