Florence of Worcester records that he succeeded his uterine half-brother in 1063 as King of Gwynedd and Powys, appointed by King Edward "the Confessor" after the defeat of Gruffydd. Orderic Vitalis records that “Blidenus rex Guallorum” joined the rebellion of “avunculos suos” (referring to “Eduinum, Morcarum”, although no family relationship has been traced between Bleddyn and Edwin and Morcar who, from a chronological point of view, could not have been his uncles) in 1068. The Chronicle of the Princes of Wales records that "the action of Mechain took place between Bleddyn and Rhiwallon, sons of Cynvyn, Maredudd and Ithel, sons of Gruffudd" in 1068, adding that "Ithel was killed in the battle and Maredudd died of cold in his flight, and Rhiwallon son of Cynvyn was slain" and that "then Bleddyn son of Cynvyn held Gwynedd and Powys".
The Chronicle of the Princes of Wales records that "Bleddyn son of Cynvyn was killed by Rhys son of Owain" in 1073. The Chronicle of the Princes of Wales records in 1106 that "Bleddyn and Rhiwallon, sons of Cynvyn, were brothers, from Angharad daughter of king Maredudd". 1
He was the son of Cynfyn ap Gwerstan, otherwise unknown, and Angharad, widow of Llywelyn ap Seisyll (d. 1023 ), and mother of the famous Gruffudd ap Llywelyn (d. 1063 ).
Late authorities supply Gwerstan with a distinguished pedigree, but the name has the air of being a derivative of the English Werestan. As half-brothers of Gruffudd, Bleddyn and his brother Rhiwallon succeeded to his domains, but no longer in independence, but as vassals and allies of Edward the Confessor. They continued Gruffudd 's policy of alliance with Mercia and aided the Mercians in their struggle with William the Conqueror, supporting Edric the Wild in 1067, when they ravaged Herefordshire as far as the Lugg, and Edwin and Morcar in 1068 .
In 1070 they had to meet a threat nearer home; two sons of Gruffudd challenged their power in the battle of Mechain, an encounter in which Bleddyn proved victor and the only survivor of the four. He was now threatened by the advance of the Normans into North Wales; in 1073, Robert of Rhuddlan established himself on the banks of the Clwyd and shortly afterwards surprised Bleddyn in a stealthy attack, in which the Welsh leader lost much booty and narrowly escaped capture.
His career was cut short in 1075, when Rhys ab Owain and the nobles of Ystrad Tywi contrived his death. The tragedy was much deplored in Mid Wales, and when his cousin, Trahaearn ap Caradog, defeated Rhys ( 1078 ) in the battle of Goodwick and drove him into headlong flight, it was held to have been signally avenged.
High praise is bestowed upon Bleddyn by the chronicle which was now kept at Llanbadarn. His virtues were those of the ideal prince — clemency, kindness, affability, liberality to the weak and defenceless, respect for the rights of the Church. Some colour is given to this eulogy by the fact that Bleddyn is one of the few princes who appear as having made amendments in the laws of Hywel the Good. To later generations he was best known as the ancestor of all later princes of Powys, for, within a few years after his death, his sons had established themselves as rulers throughout the whole of this province. 2