I think it is more than a surprise - to me it implies something unpleasant. To blindside someone in a fight is to hit them while they are looking away. If you are blindsided, it is unexpected and unwelcome. In this example, the girl has perhaps told him their relationship has finished, and he is totally unprepared for this.

Read it as, "to come in from your blind side". So you didn't see it coming, which roughly qualifies as a surprise.