Blenis, who spent more time with Manning, said Manning chose not to speak most of the time except for short, yes-or-no answers. He said Manning spurned his offers to play chess or work brain teasers by arrogantly responding, "They're a little below my level."
"I've got a person not communicating with me that's sitting in his cell, not doing anything," Blenis said.
He said he supported the brig commander's decision in March 2011 to strip Manning of all clothing at night and place him on suicide watch after Manning told another staffer that if he really wanted to kill himself, he could use the elastic waistband on his underwear.
"If someone tells me they're going to shoot themselves in the face, I'm not going to give them a gun," Blenis said.
Later Sunday, the defense showed an approximately 12-minute video clip of Blenis talking with Manning through the bars of his cell about the detainee's frustration with his situation in January 2011.
"Every day that passes by I'm getting increasingly frustrated," Manning told him.
Manning was moved in April 2011 to pretrial confinement at Fort Leavenworth, Kan. He's been held there in medium custody since then.
Earlier Sunday, the military judge said Manning's trial, previously set to begin Feb. 4, would be pushed back to sometime in March due to lengthy pretrial proceedings.
Manning is charged with 22 offenses, including aiding the enemy, which carries a maximum penalty of life in prison.