One can talk with Defender Morrill at the beginning of Operation: Anchorage, so the Lone Wanderer can come through after the elevator, though this is only for the flavor effect, as they will let you in without the perk anyway.
One can use this perk on Defender Sibley in the beginning of the Operation: Anchorage quest-line, where he will lead the player to Protector McGraw without the aggression. He still turns on the player at the end, though.
During Strike Up the Band, after helping Dean Domino with the Sierra Madre holograms, one can immediately persuade him into doing his job with the fireworks display by claiming "a man with cold feet isn't somebody I'd share a bed with."
“"Come into my parlor," said the Black Widow. You're charming... and dangerous to those of the opposite sex. Though the gender of the Commonwealth's population is about evenly split, this perk won't affect those sporting a similar undercarriage to you. Don't forget the Intimidation perk; both of these perks in unison allows almost any situation to be handled your way, without (or with) violence.”— Fallout 4 Vault Dweller's Survival Guide description
Men now suffer +15% damage in combat, and are much easier to persuade in dialogue. They are now even easier to pacify with the Intimidation perk.
Fallout: The Roleplaying Game
The Black Widow perk affects men and masculine characters, while the Lady Killer perk affects women and feminine characters—they are otherwise identical. When attempting a Charisma based skill test to influence a character of the chosen gender, the player may re-roll 1d20. In addition, attacks inflict +1d6 additional damage against characters of the chosen gender.
Dialogue choices from these associated perks do not count as Speech successes.
Behind the scenes
Black Widow derives its name from the habit of female Latrodectus (black widow spiders) to cannibalize their mates during or after mating. It is also the official FBI designation for a woman who murders her husband(s), usually in a serial fashion and/or for monetary gain.
Due to several scripting typos, in Fallout 4, ranks 2 and 3 provide no actual benefit to pacification success rates with Intimidation, and instead increase difficulty.[verified]