If "blurted" means said something abruptly and without thinking:
* Whispered: Spoken very quietly
* Muttered: Spoken in a low voice, often unclearly
* Stated: Said clearly and precisely
* Articulated: Expressed clearly and with care
* Considered: Thoughtful and well-planned
If "blurted" means said something involuntarily:
* Controlled: Kept under control
* Refrained: Avoided saying something
* Suppressed: Held back or prevented from being expressed
If "blurted" means said something rudely:
* Politely: Courteously and respectfully
* Tactfully: Sensitively and diplomatically
Other possible options:
* Reflected: Thought about before speaking
* Hesitated: Paused before speaking
* Delayed: Put off saying something
The best antonym will depend on the specific situation and the meaning of "blurted" in that context.