Fatigue cant be measured by any blood test I am aware of. What is certain is that a fatigued aircrew who have any alcohol remaining in their system is going to be impaired. Its a compound problem where one exacerbates the former.
Alcoholism is certainly a disease, and needs careful treatment. Alcoholics just get used to the distorted perceptions of being well over the limit for almost any activity, be it driving or flying. It is definitely unsafe in any case. Perversely, a person well used to alcohol may well appear to be quite OK, especially in a morning, when the colleagues are worse for wear.