It shows whether there is any blood in your alcohol or not

Seriously:
it heavily depends on what you do with the blood.
It definitely is not possible to proof paternity with normal blood tests (it is only possible to exclude paternity), paternity can only be shown with a reasonable security by doing a dna test.
It is also not possible to check for substance abuse which took place more than about a week ago (the kidneys clear out the drugs...), but in hairs many substances get builtin if they are in your blood. How far back you can proof use/abuse of a substance depends on the length of your hairs and its speed of growth (usually about 1cm (0.4inches) a month, so measure your hair to see how far back a test might reach).
It is possible to see whether you have had some infections (as the antibodies will be in your blood if you have)
It is also possible to proof your use of some substances in the near past (alcohol, drugs, ...)
For details, ask a doctor you trust.
MudGuard