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What can I substitute for freeze dried blueberries? Will frozen blueberries work? This is for a bread filling.

No, they aren't a direct substitute. Freeze dried blueberries are very dry and will absorb moisture from the dough in order to re-hydrate, frozen ones are not dried or dehydrated at all so will add water to the dough.

They also have a very different consistency: thawed from frozen blueberries will be squishy and tend to get blended into the dough, dried blueberries are tough little beasties and will hold together much better. You may be able to get around that by keeping the berries frozen and working them in at the last minute - keep in mind most frozen blueberries will have lots of water ice mixed in which you don't want.

If you want to use frozen then you'll need to account for the extra moisture in the blueberries, taking the right amount of water out of the recipe. This site says:

Dehydrating them naturally, we can use up to 7 pounds of blueberries
to make 1 pound of dehydrated blueberries.

This means that blueberries lose about 85% of their weight when dried, so if the recipe calls for 100g of dried blueberries you'd need to add 700g of frozen ones and take 600g of water (600ml because 1ml of water = 1g) out of the recipe. If you are using volume measurements then this site says 80g of freeze dried = 2¼ US Cups.

Additions to GdD: You likely would get a very different look and taste even if accounting for liquid and volume.

The freeze dried berries are going to usually be more concentrated in flavor, giving more a berry pop I would call it when eaten. Fresh or frozen, not so. As you mix and cook there flavor will spread along with the juice and you will loose that burst of flavor and instead get a subtler, more spread out flavor in all likelihood. It could even be lost in the overall bread flavor.

Second, the freeze dried are basically encapsulated, and unless you actively re-hydrate them first should not overly react with the rest of the dough. Along with keeping the flavor concentrated, this should mostly keep the color there too. With fresh berries you with get breakage and leaking and tend to color your dough with smears of blue/purple that many people would find off-putting. Frozen would be worse on this as blueberries I have always found to be one of the fruits especially prone to freezer cell damage. As soon as they start to thaw they start giving off large amounts of juice. This will not only spread the flavor, it will heavily stain the dough during mixing, rise and baking.

I personally would not be off-put by the color, but many would and appearance is often an important part of eating experience. They are some complication in addition to throwing a wrench into the liquid ratios of the recipe though.