The name is often said to be from Welsh ‘plu maen’, meaning parish stone. Problem with that is Welsh ‘plu’ means feathers! Parish is ‘plwyf‘
Cornish ‘plyw’ (pronounced, I think, ‘ploo’) means parish, and that fits the bill better. But Cornish for stone is ‘men‘
We’re left with either Welsh plwyf maen, or Cornish plyw men. But neither are a million miles away, and ‘parish stone’ looks plausible.
The problem now is establishing how early this “parish” was. First mention of parishes in English was in ‘Life of Cuthbert’ by anon of Lindisfarne, circa 700. Its “parrochia of Osingadun” was in N. Yorks, and held by Abbess Aelfflaed, but is now lost.
Ths derivation from Welsh seems unlikely (Cornish even more so). No parishes existed when Welsh was last spoken here, the Celtic church worked from monasteries. Welsh for parish stone wd be carreg plwyf -maen plwyf is unlikely, plwyf maen is very unlikely as adjectives generally follow the noun.
Most likely to derive from old anglo-Danish Bla Man which means Grey/Lead Coloured stone. Supported by Blue Beck and Blawath Beck close by-it wd make more sense for these to mean the grey becks rather than parish becks. Modern Danish still has the word Bly for lead. Grey Stone is exactly what the Blue Man is. Man unlikely to derive fm maen , English has hardly any old words derived fm Welsh, man is widely used for prominent cairns/standing stones. Found in other languages too eg Gaelic-the Fhir Duibhe on Ben Eighe means The Black Men these are prominent pinnacles on the summit.