Confidence: It's likely to be this, but I can't be certain.
Notes: This appears to be blue-sorediate, that if it were L. rupicola would make it var. efflorans, but that is rare and the thallus looks too thin. Most likely it is the asexual state of P. tuberculosa, which is common on gravestones and nutrient-poor hard rock generally, and has a surface that becomes blue to blue-grey sorediate. However, the definition of the photograph frankly is not very good, and there other blue-sorediate lichens to be found on gravestones.