A blend specification needs three things a single-component specification does not: the recipe with a tolerance on each component, a component-level quality standard for every ingredient, and the sampling rule that says how the mix ratio is verified. Without the third, the first two are unenforceable, because a 500 gram grab from one bag will never match the recipe exactly.
State the recipe as declared percentages by weight with a tolerance, for example 40 per cent peas, 30 per cent carrot dice and 30 per cent green bean cuts, each plus or minus 5 percentage points. The tolerance is not a courtesy; blending equipment meters components volumetrically and the ratio in any single pack varies. Choose the tolerance you can live with on the label, because the percentages drive both the ingredient list order and any quantitative ingredient declaration on a retail pack.
State the verification method explicitly: how many packs are sampled, whether the ratio is assessed on a composite of several packs or per pack, whether the assessment is on frozen or thawed product, and who separates and weighs the components. Composite sampling across a defined number of packs is the normal approach and it is the only one that gives a reproducible answer.
Then attach a component standard to each ingredient: calibre band, defect maxima, extraneous vegetable matter, colour reference. Components are not interchangeable and a mix is only as good as its weakest component, which is usually the cheapest one.
Two more clauses earn their place. Piece size compatibility, because components with very different sizes and densities segregate in the bag and in the case, so the pack the consumer opens does not look like the sample you approved. And cooking behaviour, because components with different blanch levels and thermal mass will not cook evenly from frozen; that is checked by cooking the blend, not by inspecting it.
Finally, name the change control rule. Component origin substitution mid-season is normal in this category and it must be a written approval rather than a discovery.