Thanks for your question about blueberries in peal gravel. Although various kinds of mulch are helpful for spreading around blueberries, the most beneficial form is one that compliments the acid-loving berry plants. So, we typically mulch blueberry plants as explained here:
"Blueberries grow better when mulched. Mulching keeps the soil cool, conserves moisture, adds organic matter to the soil, improves soil structure, and aids in weed control.
After planting, apply a mulch of Douglas-fir sawdust or bark to a depth of 2 to 3 inches. Increase the depth of the mulch to 6 inches over a period of years. You can mulch the entire soil surface (you’ll no longer have to cultivate), or you can place a 3- to 4-foot-wide band of mulch in the row.
You may have to apply 25 percent more nitrogen fertilizer on mulched plantings compared to unmulched plantings, depending on how fresh the sawdust is. Fresh sawdust “ties up” nitrogen while it decomposes, so you need to add more for the plants."
The problem with gravel is that some types of rock 'leach' when water is passed over them, which is explained in this article. If the rock is of a mineral type that is not acidic, the leachate (what goes into the soil) can make the soil less acidic (alkaline), and be exactly the opposite of what blueberry plants need.
I hope this is helpful to you. Good luck!