The fastest way into Mosaic 1, JetBlue's base elite tier, is a status match. Hold 
Alaska Atmos Gold (MVP Gold) or 
Southwest Rapid Rewards A-List — both US-based programs — and JetBlue will match you in directly. The match isn't a permanent grant on day one: it opens a three-month challenge window in which you need to earn 10 tiles to lock the status in, the same bar JetBlue sets for reaching Mosaic 1 on your own.
If you'd rather not lean on outside status, JetBlue also sells the door: the 
JetBlue Plus Card and JetBlue Business Card, both issued by Barclays in the US, earn one Mosaic tile per $1,000 you spend in a calendar year on either card. Fifty tiles clears Mosaic 1, so $50,000 of spend gets you there with zero flying required. It's slower to hit than a status match, but it isn't a one-time trigger — you can requalify the same way every year, while the airline-status matches only run once per membership.
Mosaic 2, the silver tier, is more straightforward if you're coming from Delta: a Platinum 
Medallion match is open to travelers anywhere, with no challenge period or extra catch attached to it. Between the two silver-tier paths, that's the cleaner one — Alaska and Southwest only get you to the entry tier, while the Delta match jumps a level higher for the same kind of one-time move.
Miles