1) Many a flower is born to blush unseen. 

2) The trouble began in Laredo. It was the Llano Kid's fault, for he

should have confined his habit of manslaughter to Mexicans. But the

Kid was past twenty; and to have only Mexicans to one's credit at

twenty is to blush unseen on the Rio Grande border.

请教老师，以上2句中都出现了is to blush unseen: 

is to 是什么用法？

unseen是主语补足语=表语吗？

麻烦老师具体分析一下语法和成分，谢谢！