I have heard about the Black grass found in a supplier of wildflower seeds and I believe they have recalled all their stock of same. I attached an example of the type of certification that I have for all seeds that I import from the Uk. I also requested a response from my suppliers Bright Seeds and have copied it below. This is as much of a guarantee that I can give for now other than to state that I am dealing Bright’s for about 15 years without problems of any description including invasive species and they are an extremely reputable company. I have been over there many time on training days and farm walks etc and they have an exemplary set up.
All certified seed is obviously clear and inspected, Although is not full proof to NO Black grass. 99% pure can still hold a tiny amount of Black grass as this can be carried in by birds and machinery.
There is no official certification scheme for wildflower seeds as yet. However most of the species in your wildflower mixtures would be certified by ourselves and DEFRA but there are a few that have no certification scheme like Cornflower and poppies for example.
These are seed crops harvested from our own production and we have our own stringent crop inspections for any Black grass and invasive weeds. This is as far as we can take any precautions for contamination and nothing else can be done to purify the seed.
I hope this information above is of use, we are taking more care over our seed quality than most companies because some are not officially regulated.
There is much more of a chance of Black grass coming in through bird feed and seed. There is no certification or standards for garden bird feed and as I said these seeds can be carried easily by farmland birds.