Hi Lummox,
I'm interested to know what the new price was! Four pence three farthings? Or five pence maybe?!!
Can't see the 'black out' idea working really, as the tin that is left is a bright pale yellow! Surely they weren't as late as 1939, in iron too! Price change seems the most likely option to me.
Not sure about the cost of black enamel against blue either!
As the signs have been clearly enamelled with one colour and price, then over-enamelled with black, I'm sure it must be to do with a price change. Perhaps black enamel is more opaque than blue, to hide the white lettering beneath?
I'm just going out now........where's my anorak?