Kurt had been having a rough day. He'd taken a slushie to the face (a freshly moisturized face, at that) early on and had to change his entire outfit. Now, just moments before Blaine arrived, he'd been harassed again by Karofsky and his delinquent jock friends. When Blaine surprised him, his first reaction was to jump, thinking it was the start of another torment session. He still had bruises on his arms from last time, he and the lockers weren't friends.
Hearing Blaine's familiar voice made him relax and saved the other young man from a scathing remark and/or a quick shove away. He let out a breath of relief, turning with a soft smile, "What are you doing here?"