What is the best bark for blueberrie: Fir, Cedar, or Hemlock
Thank you for your question about mulching blueberries.
In the Extension publication "Growing Blueberries in Your Home Garden" https://catalog.extension.oregonstate.edu/ec1304, Douglas fir bark or sawdust is recommended for mulching blueberries. On the other hand, a Cornell publication https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&ved=0ahUKEwi... recommends any softwood sawdust, or pine bark. Since Douglas fir is a common softwood here, while pine is more common around Cornell, it appears likely that any of the softwoods you mention would work fine.
Berry expert Bernadine Strick states: "The objectives of organic mulches such as douglas fir sawdust are to add organic matter to the soil over time, help conserve soil moisture, moderate soil temperatures and help control annual weeds. Blueberry plants grow best in an acid soil (pH 4.5 to 5.5) and like high organic matter. The pH of douglas fir sawdust is about 4.2. There are other low pH mulch options including bark mulch (a great option), wood chips, and pine needles -- all have a relatively low pH. Note that if you use a material like wood chips, you will need to watch the plants carefully as adding a more woody mulch like chips to the soil surface means you'll have to add more nitrogen fertilizer (these products take a longer time to break down and the bacteria that break down these materials use nitrogen meaning the plants have less available for their growth)."