Marvel's Black Cat has long been considered a shameless ripoff of the character Catwoman, but the DC fans' accusations, while perhaps warranted, are not based in fact. While Catwoman debuted much earlier than Spider-Man's most elusive love interest, the Marvel editorial team at the time based Black Cat on an entirely different character (from a different medium, no less). Nevertheless, rumors spread faster than facts, and thus the true origin of Black Cat ought to be explored in greater detail.
Felicia Hardy first debuted in Amazing Spider-Man #194 in 1979 as a villain out to avenge her father. Training herself to become an expert in acrobatics and fighting skills, she crossed paths with Spider-Man on numerous occasions, and their relationship become romantic. Oddly enough (once she discovers her partner's secret identity), Black Cat prefers Spider-Man to Peter Parker - perhaps not unlike a similar relationship in DC Comics between Batman and Selina Kyle, Catwoman.
Much like Black Cat, Selina Kyle is a cat-themed burglar and acrobat, calling herself "The Cat" in her first appearance in Batman #1 in 1940 (nearly forty years before Felicia Hardy's debut). Establishing herself as a femme fatale foe, Catwoman is now more often seen as an anti-hero and the most consistent romantic partner for Batman (his others, like Vicky Vale and Silver St. Cloud, rarely last a decade). Black Cat learned Spider-Man's identity fairly early on, but Catwoman's knowledge of the man behind Batman's mask varies with the author (and the specific version of the constantly-rebooted DC Universe).
The similarities between characters are admittedly close enough to warrant some suspicion on the part of DC and Batman fans. But writer Marv Wolfman never had Catwoman in mind while writing Black Cat, but rather "Black Luck Blackie", a 1949 cartoon cat courtesy of Tex Avery who would cause those around them to have bad luck. This ties into Black Cat's superpowers; though not explicit (and various versions of the character strip away her powers), she can also influence luck to a certain degree, though whether this is a conscious act or not is largely dependent on the writer.
Black Cat is slowly making a resurgence in popular Marvel media, from Sony's 2018 Spider-Man PlayStation 4 video game to taking on larger roles in the comic storylines. Catwoman has as well, with her own series and an appearance in 2022's The Batman in March. But when it comes to origin stories, Black Cat has more in common with a cartoon character than Catwoman.