I read an article back in the 90's that said that vermiculite, and to a lesser extent Perlite, retained minerals and could become toxic over time. Prompted by this discussion, I have been looking to find that article to no avail. The gist of the article from the 90's was that either absorbs minerals from the soil and water. The minerals, especially calcium and phosphorus build up in the outer layers of the perlite/ vermiculite pieces. This is especially true of the pieces near the soil surface. They can become toxic to plants over time, and can harm the base of the plants, and surface roots.
However, I did find this:
https://www.sciencedirect.com/...
The gist of this one is that vermiculite can release aluminum into the soil in an acidic environment. Aluminum is very toxic to plants. Because of this, it would not be a good addition to the soil, or mulch on the top of the soil, for blueberries. Blueberries like a low pH. Perlite might be OK as a mulch, but it would wash away given the first thunderstorm.
I stand by my recommendation of newspaper covered with pine bark nuggets as being the best way to mulch blueberry bushes.
