Lots of things can be tested for in the blood but routine/normal blood tests are really more of a general measure of health and look to see if anything is out of the ordinary, for example like inflammation going on somewhere in the body.
At the moment there isn't a blood test that can reliably pick up every type of cancer. Sometimes things that are measured in routine blood tests can be outside of the normal range when someone has cancer, but not all of the time, and lots of other medical conditions can cause abnormal readings too. Blood tests need to be considered in the context of whatever else is going on.
Do talk to your GP again if you have unexplained symtpoms that are a concern.
