Blind tasting is considered to be the gold standard of wine evaluation. And the reason for its status is that it allegedly blocks the biases that come from any preconceptions we might have about a wine. There is no question that our prior beliefs about what we taste can influence and distort our perceptions. If we know and admire the producer or know the wine we are tasting is costly we might view the wine in a more positive light. The influence of confirmation bias and other mistakes in reasoning are well documented.

It stands to reason that if we can block these potential biases through blind tasting our evaluations are more likely to be accurate. They prevent us from honestly assessing what our perceptions tell us. But that is only true if in fact blind can succeed at blocking prior beliefs. And that does not seem to be the case. Blind tasting blocks some ways of acquiring prior beliefs about a wine. But as philosopher Jonathan Cohen argues:

What distinguishes blind from sighted tasting is that the former prohibits the taster from employing specific sources of information about the perceptual object (say, from the shape of the bottle, the words on the label, testimony about the methods of production). But this leaves it open that the blind taster might come to hold the very same beliefs about the perceptual object by other means — specifically, as a result of perception and perceptually informed inference — and that those beliefs might subsequently affect her perceptual experience.

What Cohen has in mind is this:

Suppose I dislike Merlot and suppose, unbeknownst to me, the wine I am blind tasting is, in fact, a Merlot. If I am adept at blind tasting, I will correctly judge the wine is a Merlot and my prejudice against Merlot will still then shape my judgment about its quality. In this case, a prior belief is still negatively affecting my judgement about wine quality. Yet I am forming my distorted judgment based on evidence that blind tasting allows—my inference that it is a Merlot. The reasons for wanting to block a bias from influencing my judgment when tasting with no blinders are also reasons for wanting to block bias when tasting blind. So blind tasting fails to prevent my biases from informing my judgment.

Cohen also describes another case in which blind tasting fails.

It has been well established that our perceptual responses to a stimulus are influenced by contrasts with other stimuli we experience at the same time. In the context of wine tasting, how we perceive a wine is influenced by other wines we taste it with. Blind tasting has no ability to block this kind of distorting influence in which our impression of a wine is influenced by something other than that wine.

The only way to block the influence of perceptual contrast is to taste something neutral, such as water, between sampling each wine to be blind tasted. This, of course, is easily accomplished but I doubt that blind tasters are always careful enough to cleanse their palate after each wine.

Thus, blind tasting is not a comprehensive solution to worries about biases.