As soon as he saw who it was, Blaise paused and took a second to blink. While he should have figured his mother would react in this manner, he didn't expect her to just show up and find him, so he was caught off guard. But once it had registered and he realized that it was normal of her to act this way, he was as calm as he was before and all went over rather quickly.
He didn't jump, afraid of being caught in an almost compromising position or situation. Blaise's mother knew plenty of details about his life and never seemed to care what he did as long as he was well and happy, so he was actually comfortable with such things. Thus, he leaned back fluidly and simply crossed his arms over his chest. The only awkward part was Theodore sitting on his bed as his mother entered without even introducing herself, which was very uncharacteristic of her. But then again, her only son was in danger, albeit over 12 hours ago.
"I was attacked by a stranger last night, coming home from a bar. I didn't expect such violence so I wasn't ready to defend myself. He knew that I was a Death Eater and even that didn't stop him," Blaise shrugged as if it was nothing. He liked the shortened version of the story, 'I was attacked', but he knew better than to leave out details. It was best to tell her everything so that he wasn't bombarded with questions afterward.
"Mother, this is Theodore," he said, taking the introductions into his own hands, hopefully giving her something else to worry about.