A supplement panel that lists a blend has reached the limit of what the label discloses. A proprietary blend is a group of ingredients shown under a single heading, either a plain one or a name the company chose, and federal regulation requires only the weight of the blend as a whole. The amount of each ingredient inside it does not have to appear anywhere on the package. A blend listed at 500 milligrams could be 480 milligrams of one inexpensive ingredient and 20 milligrams divided among the five the bottle is named for, and that label would be complete, accurate and compliant.
A consumer reading it closely could not tell. Ingredients inside a blend are listed in descending order by weight, so the order carries information. The first is the largest share and the last is the smallest, which means an ingredient a product is marketed on that appears at the bottom of its own blend is a smaller share than everything above it. Companies have reasons to protect a formula they paid to develop, and a blend on its own says nothing about the quality of the product.
The gap between what the label states and what a buyer would need in order to compare two products is written into the regulation itself. Where an amount appears next to an ingredient the buyer knows the quantity, and where a blend appears the buyer knows a total and an order.
Source: 21 C.F.R. § 101.36, subsections (c)(2) and (c)(3). https://www.ecfr.gov/current/title 21/chapter-I/subchapter-B/part-101/subpart-C/section-101.36
