The hardest colour for a panel to get right
Blue is where cheap displays come apart. The eye has the fewest blue sensitive cone cells, so manufacturers can spend the least on blue accuracy, and the blue subpixel is usually the first to degrade with age on an OLED panel. The practical result is that a full screen blue will show banding, dithering patterns and blotchy uniformity on displays that look flawless on every other colour.
Look at it from a normal working distance rather than close up. What you are looking for is not individual pixels but large scale unevenness: patches that read purple or cyan rather than pure blue, and visible steps rather than a flat field.
Isolating the blue subpixel
As with red and green, this screen lights only one of the three subpixels in every pixel. A black dot here is a pixel whose blue element has failed, which on normal content shows up as a yellow tinted pixel rather than an obviously broken one. Those are easy to miss for months and impossible to miss here.
This is not the Blue Screen of Death
If you were looking for the Windows stop error, this is not it and nothing here will cause one. The genuine article is a system crash screen with a stop code on it, and it is produced by the operating system, not a web page. If your machine is showing one, write down the stop code, because it is the only part of that screen that tells you anything useful.
Blue light and sleep
Blue light in the evening does suppress melatonin, which is the mechanism behind every night mode feature on every device. A full screen blue at maximum brightness is about the least sleep friendly thing a display can do. Use this page for testing rather than for illumination, and if you want a screen as a lamp at night, use the red screen with the brightness slider low instead.