In the history of the Blevins/Blevin/Blethyn/Bleddyn name, one of the more famous characters in the past thousand years would have to be William Blethyn, Bishop of Llandaff, who lived in the 16th century. There is a fair amount of information on him in out of print books, many of which can be found on Google books. At a later date, I’ll go back and give a better biography of him.
The one piece of historical information that is by far the most valuable, and in my opinion, of great importance to Welsh national heritage, is the William Blethyn Pedigree Roll kept at the Glamorgan Archives. A zoomable copy of the Pedigree Roll can be viewed here. One of the amazing things to me is that this roll purports to trace the lineage of William Blethyn back through the Welsh kings of antiquity and on to Brutus of Troy, the legendary founder of Britain. And even more astounding, the top left corner of the roll take the genealogy from Brutus all the way back to Adam. I’ll leave it to individual opinion how much of the genealogy presented is to be accepted as fact.
Copyright Glamorgan Archives (Reference CL/PED/1)
At some point in the future I’d like to transcribe the text of the roll and redraw the arms listed and research them one by one until I have a clearer picture of what all is here. Given the magnitude of information listed, this could turn into a life-long project. If there is anyone else out there interested in helping, please let me know.
[Update 24 February, 2017: It has been brought to my attention that the link to the pedigree roll on the Glamorgan Archives website is no longer functional, and that the Archives present staff does not appear to be aware of the document. We can only hope that it is still in safe keeping.]
Jeremy B. Blevins 
This is amazing thank you for posting it.
In July 2023 I ordered a digital copy of the Pedigree Roll of William Blethyn for 12 pounds British (about $15.46 US). You can contact the following email address to start the process. They will email you a digital copy of the pedigree roll which you can zoom in to see the information. You could also print it out to frame and put on your wall.
[email protected]
William Blethyn was my 9th Great Grandfather.
Thomas Krauser