A blue-nosed automatic?

Here's a forum discussion on suspect is armed with a blue nose revolver that failed to come to any conclusion. But we here at ELU are made of sterner stuff...

This Wikipedia article on the Colt Detective Special revolver says

The Detective Special was initially available in both bright blued and nickel finishes...

I doubt Agatha Christie expected many of her readers to get that reference. She may simply have used it for artistic effect - "blued steel" implies heat-treating, which can either be taken to mean it's a higher-class weapon that's been more carefully manufactured, or maybe that it has been used (the blueing by implication arising from the heat of firing).