The sound “BL” implies a relationship with eyes: “blink” is two eyes closing and opening—“blind” is when they never reopen. The colors “blue” and “black,” and the hair color “blonde” all start with “bl.” To bleach is to remove color. Color and its absence can only be perceived by the eyes. Eyes can be simulated graphically by the shape of the letter “B.” The shape of the letter “L” simulates reflection: color is a reflection of light waves.
A consonant blend usually incorporates a meaning and its conceptual opposite, as black and blonde are.
The sound “bl” is also about an event that happens quickly and obscures one’s senses: a blaze, a blizzard, the blossoming of flowers, the blaring of music, things blowing up rapidly.
A block is a rectangular shape that can provide structure, or keep one from getting through or seeing, just as a blouse blocks one from seeing a woman’s bra or breasts. To block one’s sight is a method of blinding.