The pair posed for a photograph on Thursday (21 April), sharing the pic via Snapchat, and they even did a face swap as Kylie captioned it: "When we've been best friends the whole time."
Blac shared a similar photo, saying: "Hanging out with my lil sis."
Reality star Kylie's admission stunned fans as many believed she was far from good friends with the ex-fiancée of her boyfriend Tyga - and soon to be newest member of the Kardashian clan.
They have a pretty complicated history. Blac, 27, shares a son with Kylie's rapper boyfriend Tyga, to whom she was once engaged.
But Blac, who is best friends with Amber Rose (who is the ex-girlfriend of Kylie's sister Kim Kardashian's husband Kanye West), is now engaged to Kylie's brother Rob Kardashian.
There have always been reports of an ongoing feud between the two given their complex family relations. US reports had also previously claimed that Rob's relationship with Blac had caused friction within the Kardashian-Jenner clan.
Rob proposed proposed to Blac earlier this month with a seven-carat diamond engagement ring, just 10 weeks after they went public with their relationship.
It was only on Thursday that Rob set tongues wagging that they've secretly married after using the word 'wife' on social media.
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