In double blind studies neither the test subject or the reasheacher knows who was given the drug and who was given the control. why are these studies effective for testing a drug that may elevate mood and relieve feelings of depression

Because there will be no bias if the reasearcher knows which drug is being given and the subject wont know what they are being given so both may assume that the subject is going to feel happier and expect to have an elevated mood and relieve feelings of depression. Hope this helps :)