According to former astronaut Leroy Chiao the term "in the blind" indicates that you're transmitting in the expectation that the receiver can hear you but letting them know that you can't hear them.
"After the orbital debris cascade knocks out communication satellites,
Stone and Kowalski lose contact with ground control. They keep
transmitting, though, in hopes that someone is listening. Each message
begins with the rather haunting phrase, "Transmitting in the blind..."
"Yeah, that's real phraseology and that's used in aircraft operations
also," Chiao says. "You call 'in the blind' if you're not receiving
them, but you think they might be receiving you. You're basically
letting them know that you're not able to hear them."
We can also see the phrase used by NASA mission control officers in the final mission transcript of the ill-fated Columbia mission;
HOPPE: Flight, INCO, commanded string one in the blind.
HOPPE: I commanded string one in the blind, Flight.