Color film is a sandwich of four layers. From top (lens-facing side) to bottom:
Blue-sensitive emulsion
Yellow filter -- passes red and green, blocks blue
Green & Blue sensitive emulsion
Red & Blue sensitive emulsion
The yellow filter blocks blue light from reaching the bottom two layers, so they respond only to green and red light. It dissolves away during processing, leaving dyes in each layer that are formed during color development.
For "reversal" film that develops to a positive, the blue layer develops to a yellow dye, the green to magenta, and red to cyan. In a black area, all three dyes are present so no light gets through. Scraping just the blue layer in an otherwise black area of the image inhibits the yellow dye, so the magenta and cyan layers together pass only blue light -- so the scrape appears blue. If you dig more deeply and remove all the layers down to the backing, the area won't block any light and will therefore appear white.