Blue is one of the primary colors, so it cannot be created by mixing two other colors. Red and yellow are the other two primary colors.
The three primary colors, red, yellow and blue, are the only colors that cannot be created by mixing other colors. Blue mixed with red makes purple, blue mixed with yellow makes green and yellow mixed with red makes orange. Purple, green and orange are considered secondary colors because they are created by mixing only two primary colors. Tertiary colors are created by either mixing two secondary colors or mixing a secondary color with a primary color.