swear blind...

swear blind

To make a very serious, solemn pledge, especially that one is telling the truth. Janet has sworn blind that she wasn't the one to betray me, and I believe her.
See also: blind, swear

swear ˈblind (that)...

(informal) say that something is definitely true: She swore blind that she had not taken the money, and I believe her.
See also: swear