Third-person singular simple present indicative form of blacken.(verb)
Wiktionary.org : Text is available under the Creative Commons Attribution-ShareAlike License
Use "blackens" in a sentence
"The very word blackens before our eyes with necromantic characters -- again we plunge into its gulfs desirous of what we dread -- again, "in pleasure high and turbulent," we climb the cliffs of Cairngorm."
"Quite roughly speaking, as may be deduced from what has already been said on this subject, the luminous flame of acetylene "blackens" a ceiling at about the same rate as a coal-gas burner of the best Welsbach type."
"Next up is Thomasina Miers' roast tomato salsa, from Mexican Food Made Simple, which blackens the tomatoes, garlic and chillies in a searingly hot pan first."