A blood culture is a test that looks for the presence of bacteria, or less commonly fungi, in the bloodstream. Whether or not you eat before this test will not affect the result. If bacteria is present in your bloodstream, the presence of food in your intestines or elevated glucose in your bloodstream will not change the fact that bacteria are there.
I wonder whether you are having blood cultures drawn to look for bacteria, or whether you are having another type of blood test. Generally, blood cultures are collected when a doctor
suspects that a patient has a systemic, blood-borne infection, due to symptoms like persistent fever, chills, or night sweats. Other times, we check blood cultures from patients with localized infections, like pneumonia or a urinary tract infection, who we think might have a more widespread infection.
If your doctor is checking other blood tests, such as you blood count for anemia
or your kidney function, you can eat before those tests. In general, your doctor will specify if you should be fasting before a blood test. You can always contact the doctor's office or the lab to ask whether you should be fasting, just to make sure.