The point of the double blinding is specifically to counteract the unconscious biases that may occur in a single blind trial, but it can’t always be proof against someone very consciously trying to circumvent the system.

Let’s say you have a single-blind trial of a new pain killer for example. The subject doesn’t know if they are getting the new drug being investigated, an old drug that’s the usual treatment, or a placebo. But because it’s NOT double blind, the researcher DOES know which bottle contained which medication and which patient gets each one. Now even if he tries really hard to be objective and impartial, he might accidentally rate one group as limping less on the treatment he thinks is going to work best, or he might avoid giving the placebo to someone he feels sorry for and wants to make sure is getting a real treatment, and accidentally skew the groups so it’s not a fair comparison.

If the same experiment were run as a double-blind experiment, one researcher or team will divide the participants into randomized groups and pass out the medication— and if at all possible that medication will be formulated to appear identical (each dose is 2 capsules of the same size and color, for example, no matter what’s inside them), and once they’ve told the participants the instructions for how to take the meds and what surveys to fill out to track their progress and when they need to come back for rechecks with the doctor(s), they’re done for now. An entirely different researcher or team will actually collect the data, and they will have absolutely no idea who is getting what. So in order to fudge the results they would need to break into the other team’s files and sneak a peek at the list of who got what, but that’s deliberate cheating at that point, not unconscious bias.

However: you’re not wrong that— like life in general— there are a LOT of ways very subtle unconscious bias can slip into the process (who gets recruited for the study, for example— do you only advertise in certain neighborhoods? In certain languages? And so on…) I just did a search for “experimental design to avoid bias” and there was no one particular great link to point you at because there were MANY resources and webinars and publications, meant to train university and medical researchers and also to look back and evaluate any bias that might only be obvious in hindsight, the better to avoid it in future.