What is the difference between a molecule or set of molecule's half
life and the time it becomes undetectable in the blood? For instance,
if substance X has a half life of 2 days and is ingested by a person,
is there a way to know when that substance will become undetectable?

It is a function of the sensitivity of the test. The more sensitive
the test is, the longer you will be able to detect the substance. Once
you know the lower limit of the test, it is straightforward to
determine how long you will need to wait.

The previous comment is valid i,e you need to know test sensitivity
but the following may help. As you probably know half life is the time
taken for 50% of the product to decay or decompose. I assume you are
mainly interested in radioactive decay but principle is the same. The
amount of product remaining takes the following path.
Numer of half lifes       Amount decomposed
1                          50%
2                          75%
3                          87.5
4                          93.75
5                          96.9
6                          98.4
7                          99.2
8                          99.6
9                          99.8
10                         99.9

Once you get down to 0.5% of your original product I suspect most
tests would be at there limit.
Nick

In nuclear terms, as opposed to the biological terms suggested, the
standard rule for operators is 5 halflives to nothingness, and to
regulators (EPA, NRC, DoD, etc.) they usually use 7 half lives.  These
are usually applied to Co-60, N-16, Cs-137, etc.