All wood ash raises pH ("limey").

The calcium and potassium in cell walls are reduced to oxides in burning, which become hydroxides (highly alkaline) when exposed to water, but thence to carbonates when exposed to atmospheric or soil carbon dioxide (still alkaline but not as stronly so).

Blueberries will take calcium from soil (and can even show Ca deficiency I've read) but the soil calcium does not need to be in the carbonate form. Soil calcium occurring as gypsum or certain feldspars (or presumably also from rotting wood) will have comparatively little effect on pH when extracted.