Which preposition to use with blackish

on Occurrences 1%

The throat and region of the head is varied with blackish on a white ground.

with Occurrences 1%

Upper parts all freckled with brown, black, and tan color of various shades, as if sun-burnt, with a velvety black patch on each side of the neck, and the longest wing-feathers plain blackish with brown tips; top of head plain brown.

Which preposition to use with  blackish