The main difference between DNA and blood tests is one of timing: your DNA doesn’t change, but your blood does.
A blood test gives you a snapshot at the time of the test. It may reveal useful information to identify possible problems that need immediate attention, but it will not provide clues as to the causes of the problem or identify likely future problems.
Individual blood tests are best seen as items in a series – i.e. take a test, make a change, take another test to see if the change has made a difference.
Another key difference is what action you’re likely to take in response to the results of each one. A DNA test helps you focus on preventative measures – understanding your predispositions and doing what you can to make sure the harmful ones don’t manifest. A blood test is more likely to show you what’s already manifested, so your response is likely to be curative, rather than preventative.