Shortly, the letters "Bl." if added before the lab test name it refer to the word "blood", which indicates that your doctor requests measurement of analyte's concentration in the whole blood not just the serum or plasma part.
For instance, Bl. Pic. test means "Blood picture" which has another name "full blood count"
The lab tests that measured using the whole blood samples include: Blood typing, Rh factor, G6pd for favism, and Hemoglobin concentration.
However, sometimes doctors just write a "Bl." letters or the full word "Blood" before any lab test, for instance, Blood Urea Nitrogen test, a lab test that is measured from the serum part not from the whole blood.
Is this explanation essential to know before doing your lab testing?
Of course not, this is a lab deal and you shouldn't concerned about what is written before the name of your labs, it will not affect your final result if you already know the difference between "serum test" and "blood test", but the laboratory technicians already know it because that's there work.
However, some letters and shortcut forms you should be aware of them because they can help you understand your lab test results, including, the meaning of the letter "H" and the letter "L" and the mark "S" before some labs.
