Blaise should have expected it.
He knew that Draco was a coward.
A big one, always getting out of every situation no matter the price.
He also disliked Potter but that did not mean that he wanted Voldemort to win.
It was hard to see his friend walking over to the dark side.
He wished that he knew sooner what was wrong with Draco so he could have helped him somehow.
But it was too late now.
Would he have done the same? – No.
He was no Gryffindor but he would have stayed and continued to fight. He will stay.