mix-blend-mode blends an element with everything beneath it; background-blend-mode blends background layers within a single element. multiply darkens (the same formula as Photoshop), screen lightens, and difference produces an almost-inverted, high-contrast result.
Overlap the same two shapes and switch only the mode to see the difference clearly — it's the same math design tools like Figma and Photoshop use.
When to use
Common for duotones, texture overlays, and coloured shadows. Colour-space handling differs subtly by browser, so verify important screens for real.