For subjects with bright colors on a black background, if spot metering is used, the exposure parameters are set according to the brightness of the subject. In the case of underexposure in the dark, the black background will be pure black. If you shoot a dark subject, if you still use spot metering and set the exposure parameters according to the brightness of the subject, the shutter speed will be much slower, the dark places will be fully exposed, and the black background will not be so "pure black".

If you use average metering, even if you shoot a bright subject, the black background will not be so "pure black".