Double blind study means:
Blinding: 
Blinding can be done in three ways:
(a) Single blind trial: The trial is to planned that the participant is not aware whether he belongs to the study group or control group.
(b) Double blind trial: The trial is to planned that neither the investigator nor the participant is aware of the group allocation and the treatment received.
(c) Triple blind trial: The participant, the investigator and the person analyzing the data are all "blind'.
Ideally, of course, triple blinding should be used, but double blinding is the most frequently used method.
Reference: Essentials of Preventive and Community Dentisitry, Soben Peter , 4th ed page no 73
Observer is blind about the study
Person or group being observed is blind about the study
Both observer and person or group being observed is blind about the study
Interpreters and analyzer are blind about the study