Black runs

• Usually vertical surfaces, where rainwater “licks” on them along with air pollutants (mainly soot, smudges from fireplaces, radiators, grills, etc.).
  “Hydrophobic or waterproof” surfaces that do not allow rainwater to remove dirt.
  Construction failures (eg lack of marble barrier).

The shade in the corners when with a brush looks darker than in the rest of the wall where the paint has been applied with a roller.

THE SOLUTION