Below is a translation of the place name Poniou, kindly provided by Sam Rogerson from MAGA (Keskowethyans an Taves Kernewek – the Cornish Language Partnership)
‘Ponjou’ / ‘Poniou’
Initially it appears to look like ‘black bridge’. The word for ‘bridge’ is pons. The word for ‘black’ is du and it is often pronounced with a j sound instead of a d when it is stuck onto another word in place-names. The same happens with Poldhu on the Lizard, which is traditionally pronounced Poljoo.
In this case the ‘ns’ of pons and the ‘d’ of du would normally turn into an ‘nj’ sound when speaking.
Boswednack is one of the last places where Cornish was reportedly spoken traditionally.
I have no early spellings to hand of this collection of houses, apart from the First Edition OS map of around 1880, and that gives ‘Poniou’
I haven’t seen the spelling ‘Ponjou’ for that near Boswednack, but there is in Gulval Parish a ‘Ponjou’ which was earlier spelt (1343 and 1398) ‘Ponsou’, which to me is the plural. Therefore I am doubtful as to the ‘Pons’ + ‘Du’ idea although this seems to be the case at Mullion:-
Poldhu Poljew 1678.
Poldhu Cove Porthu 1493, Polsewe 1533, Poll jew 1699, Poljew 1813.
Poldhu Point Pedn Poljew 1876, Poljew Point 1888.
I am more inclined to think there has been in use two plurals of ‘Pons’ – ‘Ponsyow’ and ‘Ponsow’, which satisfies the Gulval one, and I think, may answer the one here by Boswednack.