The Red Sox should just designate Blake Swihart for assignment.
If another club claims Swihart, the Red Sox save his 2019 salary of $910,000 and open up another slot on the 40-man roster (which currently stands at 39):
https://www.rosterresource.com/mlb-boston-red-sox
If no team claims Swihart, the Red Sox can outright him to the minors without his consent because Swihart does not have three years of MLB service. However, the Sox would still owe the 2019 salary of $910,000 even if the Sox release Swihart.
Either way Swihart should get more reps than he would receive as the third-string catcher for the Red Sox.
From Cot's Baseball Contracts:
Designated for Assignment (DFA)
A player designated for assignment is removed from his club’s 40-man roster and, within the next seven days, traded, released or, if he clears waivers, assigned to the minor leagues. A club may not designate a player for assignment if the corresponding transaction is to recall a player on optional assignment.
A player designated for assignment may be traded. A club interested in acquiring a player who has been designated for assignment may try to work out a trade before the player is placed on waivers, eliminating the possibility he might be claimed by a club with a higher waiver claim priority.
A player designated for assignment who clears waivers and is not traded may be released. The player then becomes a free agent.
A club wishing to send a player designated for assignment to the minor leagues must first place him on irrevocable outright waivers, making him available to the other 29 clubs in reverse order of won-lost record.
If the player is claimed, he is lost to the claiming team for $20,000. (Irrevocable waivers may not be reversed.) The claiming team is responsible for the balance of the contract.
If the player is not claimed (clears waivers), the club may option him or assign him outright to the minor leagues, though he must continue to be paid according to the terms of his contract. A player may be assigned outright to the minors only once in his career without his permission. Thereafter, he may either 1) reject the assignment and become a free agent, or 2) accept the assignment and become a free agent at the end of the season if he’s not back on the 40-man roster. Additionally, player with 3 years of major league service may refuse an outright assignment and choose to become a free agent, regardless of whether he has been sent outright to the minors previously. A player with 5 years of major league service time who refuses an outright assignment is entitled to the money due according to the terms of his contract.
https://legacy.baseballprospectus.com/compensation/cots/league-info/transactions-glossary/