I dont' think the issue is accuracy. With cortisol, the levels vary so greatly during the course of the day--high in AM, low at night. An AM blood sample tells only part of the story.
http://www.canaryclub.org/adrenal-article-980/706-cortisol-1-or-4.html

I don't remember which company a close friend used for saliva cortisol. On the same day as the saliva test she also got a blood cortisol in the morning. They both reported similarly normal levels in the AM; however the saliva cortisol was low during the remainder of the day. Her situation was similar to the one at the site above.

this was consistent with her life. She can get some things done in the AM but struggles later in the day. She then struggles to get to sleep at a normal time.