Ptah Myers, Adauto Neto and Machel Millwood added two goals each for the Blast, and goalkeeper William Vanzela recorded 18 saves.
Game 2 is set for Saturday night at 1st Mariner Arena, with a deciding mini-game to be played immediately after regulation, if needed.
The Comets are no strangers to an opening loss, having rallied in the semifinals to beat the Milwaukee Wave in Game 2 and the mini-game.
Donatelli scored Thursday night's first goal 33 seconds into the match. Missouri's Byron Alvarez tied the game at 2, but the Blast retaliated with goals by Neto and Myers for a 6-2 lead before the end of the first quarter.
Donatelli opened the second quarter with another first-minute goal, and the Blast used scores by rookie Lucas Roque, Millwood and Neto to pull ahead 14-2 midway through the period.
The Comets fought back in the third quarter, outscoring the Blast 6-2 and cutting their deficit to 16-8.
Missouri's Milan Ivanovic opened the second half with a goal, and after Myers scored for Baltimore to make it 16-4, Ryan Junge and Lucas Rodriquez pulled the home team closer with goals.
Millwood scored his second goal in the eighth minute of the final quarter to make it 18-8, but the Comets scored the next two goals, both with their sixth attacker on the field to keep their hopes alive. Donatelli's 3-point goal with 25 seconds completed the scoring.
Blast 6 8 2 5 — 21Missouri 2 0 6 4 — 12
1.Blast, Tony Donatelli (Mike Lookingland), :33, 1. Missouri, Byron Alvarez (Vahid Assadpour), 1:20; 2. Blast, Adauto Neto (Max Ferdinand), 3:53; 3. Blast, Ptah Myers (Adauto Neto), 7:19. Penalties: None.
4. Blast, Tony Donatelli (Adauto Neto), :59; 5. Blast, Lucas Roque (Max Ferdinand), 2:30; 6. Blast, Machel Millwood, 3:40; 7. Blast, Adauto Neto (Ptah Myers), 7:00. Penalties: None.
2. Missouri, Milan Ivanovic (John Sosa), 7:20; 8. Blast, Ptah Myers (Tony Donatelli), 3:31; 3. Missouri, Ryan Junge (5:07); 4. Missouri, Lucas Rodriquez (Danny Waltman), 7:00. Penalties: None
9. Blast, Machel Millwood (Mike Lookingland), 7:59; 5. Missouri, Byron Alvarez (SAG) (Vahid Assadpour), 10:31; 6. Missouri, Leo Gibson (SAG) (John Sosa), 11:31; 10. Blast, Tony Donatelli (3PG vs6thA) (Adriano Dos Santos), 14:35. Penalties: None.
Shots: Blast: 25. Missouri 38. Power Play: Blast: 0 for 0; Missouri: 0 for 0. Goalies: Blast, William Vanzela: 18 saves, 24 shots facedMissouri, Danny Waltman: 11 saves, 20 shots faced. Attendance: 5,279