There is a saying in the wine industry - 'a look at the label is worth ten years experience'. When looking at a new wine, range of wines from the same country, region or vintage or tasting wines in a competition environment. To ensure impartial judgment of a wine, it should be served 'blind' - that is, without the taster(s) having seen the label, capsule or bottle shape.
Blind tasting may also involve serving the wine from a black wine glass (like the Riedel Blind Tasting Glass pictured below) to mask the colour of the wine. A taster's judgment can be biased by knowing details of a wine, such as geographic origin, vintage, price, reputation, colour, or other considerations.
Scientific research has long demonstrated the power of suggestion in perception as well as the strong effects of expectancies. Any knowledge that you have about a wine can cloud your judgement or influence your assessment.
Example: perhaps you don't like Cabernet Sauvignon (well to be precise - you are still to find a Cabernet that you like). So any Cabernet you taste going forward - will already have one strike against it before it even hits your lips. Maybe the wine was ultra-expensive. You may be willing to give that wine a better score card simply because it cost you an arm and a leg. These factors and many more can influence your opinion, subconsciously or otherwise. The best way to make an honest assessment is to know nothing at all.
There is another reason to taste blind. Tasting a wine blind forces the taster to concentrate on every small aspect of the wine. The taster may be struggling to pinpoint the style or origin of the wine - they will try even harder to identify aromas, flavours, winemaking or styles.
When blind tasting at a wine club or even at home with friends - do not get discouraged by any wrong answers. Take from each example a part that you did well, even if it is something as simple as finding raspberries in the aroma. Each time you will gain more experience that can be drawn upon the next time you taste wine blind.