If you have a strong family history of breast cancer or other cancers, blood tests may help identify defective BRCA or other genes that are being passed through the family. These tests are often inconclusive and should only be done in select cases after a thorough evaluation with a genetic counselor. Unless you are at high risk of hereditary breast or ovarian cancers, genetic testing usually isn't recommended.
In general, testing is beneficial only if the results will help you make a decision about how you might best reduce your breast or other cancer risk. Options range from lifestyle changes and closer screening and therapy with medications such as tamoxifen to extreme measures such as preventive (prophylactic) bilateral mastectomy and removal of your ovaries (oophorectomy).