In some cases, it may not be possible to blind participants, treatment providers, or outcome assessors.
For example, consider a study where participants are randomized to receive either surgery or a pill. It would be hard (although not impossible) to blind participants in that context!
In the same study, you probably couldn’t mask the treatment providers either. Surgeons need to know whether or not they are supposed to operate!
There also are cases where blinding isn’t as important because an outcome is not up to interpretation. For example, you can’t manipulate an assessment of whether a patient died during the study.
That’s why, when you assess this domain, you not only look at whether blinding happened, but whether it was possible or necessary.