Surprise didn't cover it. Barty froze, eyes widening with his shock at not only being recognised, but by somebody he had known in the past he was trying to escape.
"R- Regulus?" he stammered.
It was like seeing a ghost. When Barty had been in her thrall, Bella had ranted about the blood traitors she was seeking to avenge herself on, and when Barty had regained his memories he had realised that the 'Regulus' she had spoken about had been the young man he had once known. Logically that then meant that the Regulus he had thought dead was actually alive, but logic didn't work all that well when one was under stress.
He hadn't known Regulus all that well: he had firstly just been a boy in the year above him at Hogwarts, and then one of many fellows following the Dark Lord. It had been Regulus' death that had really brought him to Barty's attention, but still, even all these years later, the Black looks were unmistakable. It really was Regulus Black. And what was more, he knew who Barty was.