Is the red stuff pumice or the more commonly-referred-to red lava rock that comes in various sizes? A couple inches of either will not bother the established blueberries.

I have found that the larger grades of lava rock tend to discolor to dark or green depending on how readily moss or lichen grows in the area.

The larger the grade or size of stones, the more difficult it would be to pluck up fallen fruit; perhaps you are not doing much of that during harvest.

I would wonder if the stone product might be alkaline and therefore incompatible. Otherwise you will not rid yourself of weeding and picking up debris by going from organic mulch to mineral mulch - and as mentioned the stone material can be hard to tidy up. The only advantage is that it does not decompose. But spent plant parts will still fall on it and either have to be meticulously cleared away or a little humus layer will form that small weeds can get started in.

Weed cloth works where you are using a heavy nursery grade material and it is keep free of all debris, including intentionally applied mulches or stone coverings. Even in a nursery setting anywhere potting soil falls on it or a hole is poked in it weeds are likely to result.

I thought lava rock was pumice... Sounds like it will be ok to use. Im going to give it a try and try to remember to post an update on a few years. Any reccomendations on where i can buy it cheaply around west seattle?

Pumice is white and very light. Some of it floats.
Red rock with holes in it is lava.
Red soft 'rock' is a product of partially burned coal.
I like to use organic mulches.
I'm surprised you removed the bark mulch to put down rock. Why not leave it and then cover it with rock if that's what you want to use?
Mike

Pumice is volcanic. Not all volcanic rock is pumice. Scoria, tuffa, basalt, etc. The red coal by-product is called "clink" and is sometimes referred to as scoria.

rock mulches seem best for plants suited for high drainage/low nutrient type environments, alpine, desert etc (not really putting blueberry in that category). once the plants get established, you can bust out the flame weeder which is the most fun way to get rid of weeds.

if i were forced at gunpoint to grow blueberries in a rock mulch, i would probably try keeping the nutrients up with boxed, granular fertilizer for acid loving plants.

BBs are not grown in solid rock minerals. They are grown in soil with a small amount of rock minerals in it to increase drainage. BBs need extra water and extra drainage. Pumice does not increase or decrease pH if I remember right.