Why is 450 nm monochromatic light perceived as blue or violet depending on its intensity?
This is a general feature of human perception of blue-white mixtures. It's known as Abney effect. It's not limited to highly monochromatic blue colors (not even to blue colors). We can observe this blue-related phenomenon even with longer wavelength light than 450 nm – e.g. sRGB blue, whose dominant wavelength is about 464 nm. Here's what most people would undoubtedly call blue – in HTML notation #0000ff:

And here's a mixture of 60%† white with 40% blue (in HTML notation #cacaff):

To me, if I haven't stared at shades of truly violet a few moments ago, this mixture seems to have a considerable amount of violet — more than I'd expect looking at the primary blue. Naturally, shorter wavelength light should give even more violet perception, until it becomes violet even in its pure form.

† Percentage is given in linear scale, actual color presented is gamma-corrected to be displayed on sRGB monitors.