participants do not know which arm of the study they are in (i.e. they do not know what drug or drug dose they are getting)
A study is blinded if any or all of the clinicians, patients, participants, outcome assessors, or statisticians were unaware of who received which study intervention. The double double-blind usually refers to patient and clinician being blined, but is ambiguous so it is better to state who is blinded.
In a blinded trial, some or all parties do not know which patients received the experimental treatment. By removing the opportunity for bias, successful blinding gives researchers and the FDA a more objective look at a drug's efficacy and safety. Trials may be single blind or double blind, but these terms are often general and it is more informative to specify if the researchers, patients, outcome assessors, and/or statisticians are blinded.