Blueberry plants (mail order). 
newly planted in grow bags (September)

Plants were a bit purple when arrived but I was told they could be stress from delivery and heat.

Planted in mixture of peet moss,  pine bark and perlite.  I added a very small amount of elemental sulfur to the top of the soil. 

I’ve been watering regularly and thought maybe the purple dots were water marks causing a fungus?!?

Although fungal leaf spots can be quite common on a wide variety of shrubs, perennials, and trees, usually they are not concerning and do not affect the long-term health of the plant. In this case, the spotting appears to be physiological, which would mean it was impacted by environmental conditions and not an infection. (We don't know what particular environmental condition would be responsible...it might be high or low temperatures, a temporary nutrient deficiency, or a significant change in sun exposure.) No treatment is needed, and overall the plants appear to be in good shape.

Since gardeners sometimes don't think of watering once it gets cool, we'll mention that they should continue to be monitored for watering needs throughout winter, as plants in containers are much more vulnerable to temperature swings (as the roots are more exposed than those insulated by the ground) and the potting mix doesn't hold moisture as long as soil does (out of necessity, so the drainage is good). Fabric bags will dry out faster than containers of similar size made from plastic, so take that into account. Avoid watering unnecessarily, which could deprive roots of oxygen, so feel the potting mix about two or so inches deep for moisture first, and water only once it becomes somewhat dry to the touch at that depth. It can be easier to accidentally over-water young plants in large pots where there roots haven't filled in yet compared to older plants, but the fabric being more porous than plastic may help with that.

Potting mix won't necessarily need to be acidified with sulfur, especially if it contains a high proportion of peat moss, which is naturally quite acidic. (Sometimes it's sold already treated with lime to neutralize the pH a bit, and sometimes not.) If the potting mix didn't come with some fertilizer already in it, you could apply some to help fuel root growth (since the leaves are shedding soon, it won't do much for them until spring), or wait until spring. You can find a handful of general cultivation tips on our Growing Blueberries in Containers page.

Miri