May I ask why ancient painters sometimes signed their names with blue and black seals?

The color of the seal depends on the color of the inkpad.

The color of inkpad depends on the background color of painting and calligraphy paper.

Generally speaking, paper with white background and yellow background uses red inkpad; Yellow inkpad for paper with red background; In ancient times, blue and black inkpad was mostly used in special occasions, such as funerals and official documents.