Could i have a blood test done for everything?

No: Because many illnesses are not diagnosable by bloodwork, and there are a host of genetic illnesses for which the labs are very expensive and are a waste of money if there's no reason to suspect the illness is present.

No: There are many disorders that would not be diagnosed by a blood test. The other problem is that, if you get a whole slew of blood tests, it is likely that "something" will be abnormal. That could lead to a lot of unnecessary tests to decide if it is a true result or a false positive. It is best to have specific testing as recommended by your doctor for your specific problems.