Samuel L. Jackson says Nick Fury isn't in Black Widow, as far as he knows. As one of the original characters of the Marvel Cinematic Universe, Fury is largely responsible for everything that came after his first appearance in the post-credits scene of Iron Man. He built S.H.I.E.L.D. up to be a top-tier intelligence agency, started the Avengers Initiative and recruited Tony Stark and Steve Rogers to his cause.
As the MCU has continued to grow, Fury's connections with the larger universe and its characters have expanded. For instance, Captain Marvel showed how he came to learn about the intergalactic threats that could attack Earth and to the world of superheroes. Jackson has racked up multiple MCU appearances through it all and could keep popping up in movies set in the present day, but also prequels, like Black Widow.
During an interview with Comics Explained, Jackson was asked who his favorite Avenger is. He revealed that it is Scarlett Johansson's Natasha Romanoff aka Black Widow. But, during his answer to why she is his favorite, Jackson states that, so far, Fury is not set to be in the Black Widow solo movie.
It's Natasha who was always been my favorite because I recruited her. So far, I'm not in her origin story movie [Laughs]. She's the person I was closest to in that group.
The vital part of his answer here is how he prefaces it with "so far." Marvel Studios operates typically in secret, but they have so far not officially announced any of their plans for Phase 4. This includes Johansson's long-awaited starring vehicle that is currently filming. As part of this secrecy, Marvel has a well-documented history of calling actors into projects late in the game or at a moment's notice. This could lead to Jackson getting one of these calls, giving him a role in the movie of his favorite Avenger.
On that note, though, Jackson's reasoning for why Black Widow is his favorite Avenger is strange. He says that she is his favorite because Fury recruited her, but that is not what we know from the movies. Natasha explained in The Avengers that Clint Barton aka Hawkeye was sent to kill her (likely on orders from Fury) several years back, but he made the call to spare her life and recruit her to S.H.I.E.L.D. instead. This could be Jackson misspeaking on the matter, but it could also potentially hint that Fury always wanted Natasha enlisted and not killed. Barton is not a ruthless killer (at least not at this time), so Fury could've known that he would spare her life and sent him on the mission for that specific reason.
Regardless, it is difficult to say if there is a place for Fury in the Black Widow solo movie. The movie has to be a prequel based on her death in Avengers: Endgame, but we don't know the exact setting for it. Recent set photos indicate that it could be set post-Captain America: Civil War and before Avengers: Infinity War while Natasha is on the run and in hiding. Fury's whereabouts during this time have yet to be explored though, giving Marvel an easy path to incorporate him in Black Widow if they want.
Source: Comics Explained