The blue zone in a candle flame is blue in colour due to complete combustion. Complete combustion takes place in invisible (non-luminous) zone also. Instead of becoming blue in colour, why is it invisible to us?

The blue zone at the bottom of the wick of a burning candle is due to burning of carbon-monoxide gas produce by the incomplete combustion of carbon particles in wax.

Whereas the non- luminous zone is the outermost zone of the flame where complete combustion of wax takes place. It is faintly blue in colour but hardly visible.