A proprietary blend is basically a list of ingredients that is specific to a product formula. A manufacturer is legally obliged to state all ingredients on a products label including the amounts (in weight). The exception to this rule is if the ingredients are within a proprietary blend, in which case the manufacturers only have to mention the total weight of the blend (grams or milligrams).
Things to look out for…
Scrutinise any proprietary blend that contains a high number of ingredients, but a low total weight e.g. a blend that is just 5g in total weight, yet contains creatine, beta-alanine, l-arginine and caffeine are unlikely to deliver the requisite amounts of each ingredient in order to exert a beneficial effect! Creatine alone should be 5g at least, so a proprietary blend whose total ingredients total 5g is not going to deliver anywhere near enough to fit purpose. The consequence of all this is that you are paying for something that may not necessarily be delivering the quantities of each key ingredient that you need!
Most supplements out there are pretty good and deliver optimal ratios of each ingredient, but it’s prudent to be aware of what terms such as ‘proprietary blends’ actually mean and how you can fully utilise your money and supplements.