Short answer
In a home facing a busy street, windowsills get covered in dark soot. This comes from exhaust soot and brake and tyre dust. Because these particles are very fine, they pass easily through an ordinary insect screen.
Why is it darker?
Roadside dust is different from soil dust: it contains soot. That is why curtains turn grey, white surfaces get dirty quickly and are harder to wipe clean.
Made to measure: hook-and-loop or roll

