Is the 2018 Black Lightning TV show considered part of the Arrowverse franchise (consisting of Arrow, The Flash, Supergirl, Legends of Tomorrow, Constantine, and several web series)? It airs on the CW channel and has Greg Berlanti as Executive Producer, like the other DC Comics superhero shows. Does that mean that it's part of the same shared multiverse?
As of now, no.
According to this io9.Gizmodo article, Black Lightning will not be part of the Arrowverse, at this time.
“We do not aim to do a five-way crossover,” Pedowitz said. “Black Lightning, at this time, is not part of the Arrowverse. It is a separate situation.”
It is unclear what the reasons for the decision were. One possible reason for the separation is any cross-overs would require moving the team from the Arrowverse base of operations in Vancouver, to the Black Lightning base of operations in Atlanta (or vice versa).
In another interview with Black Girl Nerds, Pedowitz reveals similar sentiments:
It was different from all of our other superhero shows and it was never designed to be apart of the Arrowverse.
BGN Exclusive interview with Mark Pedowitz, President of the CW - Black Girl Nerds
This is re-iterated by executive producer Salim Akil in an interview with the Wrap:
“I say this with all due respect, but they’re not really relevant to the show that we’re doing,” executive producer Salim Akil explained at the Television Critics Association press tour on Sunday. “The great thing that Warner Bros. and CW allowed us to do was create our own world. We really wanted folk to get to know this family before we started branching out.”
‘Black Lightning’ Executive Producer Explains Why Newest CW Superhero Isn’t Part of the Arrowverse - The Wrap
And finally, according to Greg Berlanti in an interview with Variety, he responds to the question of "[Black Lightning]'s going to exist outside the Arrowverse, correct?" with the following:
Yes. As far as I know everyone’s always wanted that. Salim wants that. And the studio and the network want that. They’re very different tonal shows. As people watch them. I’m not sure they could sort of ever quite imagine yet how we would be able to ever connect those things. It’s always existed outside.
Greg Berlanti Opens Up About Andrew Kreisberg: ‘I Was Shocked, I Was Disappointed, I Was Confused’ - Variety
While from the above it is clear that it is not part of the Arrowverse, executive producer Salim Akil has definitely thought about which Earth number Freeland would have:
"You know it," he said with a smile. "You know it. Yes."
'Black Lightning' EP Salim Akil, Star Cress Williams on 'Arrowverse' Crossover Potential
And on a side note (as pointed out by @Thunderforge), it would appear that Black Lightning featured a Big Belly Burger; as you’d expect given season 2 of the Flash, when Hunter Zolomon (posing as Jay Garrick) remarked "Every Earth has a Big Belly Burger".