The phrase "bluish color" means a color that is similar to blue, but not exactly blue. It is a slightly different shade or tint of the color blue. Full definition
The vet may also point out other signs to you such as bluish coloring of the gum or swelling especially of the abdomen. (uvma.org)
The second plane cuts from the surface to 24000 km deep showing areas of faster sound speed as reddish colors and slower sound speed as bluish colors. (space.com)
They betray their presence by their conspicuous bluish color and by softening the outlines of buried craters and canyons (inset). (sciencemag.org)