A proprietary blend is a list of ingredients grouped under a single combined weight, with no breakdown of how much of each is inside. The label might show a blend totaling five hundred milligrams listing six ingredients, but it will not tell you whether that is a near-even split or four hundred ninety milligrams of the cheapest one and a sprinkle of the rest. That missing breakdown is the red flag.
The reason this matters for a buyer is comparison. If you cannot see the amount of each ingredient, you cannot compare that product to a competitor that does disclose amounts. You also cannot do normalized cost-per-serving math on any single ingredient, because you do not know how much of it you are getting. The blend converts a comparable product into an uncomparable one.
Ingredients on a supplement facts panel are listed in descending order by weight, and that ordering is the one clue a blend leaves you. The first ingredient in a blend is present in the largest amount, the last in the smallest. So if the ingredient a product is marketed around appears last in its own proprietary blend, that ordering is telling you something the marketing is not.
None of this means a proprietary blend is harmful or that the company is doing something wrong. There are legitimate reasons a formulator might not publish exact ratios. The point is narrower and entirely buyer-side: a blend is a place where the label withholds information you would otherwise use to compare, and you should treat that withheld information as a cost.
A practical move at the shelf is to set blended products and fully disclosed products into two piles. Within the disclosed pile you can do real comparison. The blended pile you can only judge on price and trust, never on per-ingredient value. If two products are otherwise similar and one discloses its amounts, the disclosure itself has value.
Whether any of these ingredients is worth taking is, as always, a question for a qualified healthcare professional. The blend question is purely about transparency: are you being given the numbers you need to comparison-shop, or are you being asked to pay for a total weight and trust the ratio inside it? Reading the panel is how you tell the difference.