Fixing signage flush to concrete block
I would try a construction glue in a caulking-gun


Random example from Internet, any product with same characteristics will do


  
  Waterproof: Can be used in interior or exterior applications
  Versatile: For use on wood, metal, plastic, foam, mirror, concrete and brick
  




I've used a similar sort of adhesive to attach small plastic signs to rough granite, with mixed success. Some lessons - 


You need the surfaces to be clean, dry and free from dust or loose debris.
If the back of the sign is very smooth, it might be worth experimenting with roughening it with sandpaper. This may depend on the specifiic adhesive.
When fixing signs on a vertical surface, I found it best to support the sign until the adhesive sets. I fabricated a temporary support using scrap wood and other materials.