Jack
The data are from England, Select Births and Christenings, 1538-1975, which you can access via Ancestry.com.
It’s an unusual name so they could be related – and it is the only close match in the database, although records are, of course, incomplete. On the plus side, the location seems good and Holborn was in an area where opticians businesses gathered. Possibly a grandson? Or possibly the date from which Blethyn is believed to have been active is awry?
All worth further work. I’ll have a root around and see what I can find if it will help – though I’ve no wish to get in your way.
Alan.