The blue coloring on macs is due to both an acidic soil pH and available aluminum. Typically, aluminum sulfate (aka hydrangea bluing) is used - it is both an acidifying agent as well as a AL source. However, I would not use just the AS alone to increase soil acidity. You would need to use it in too high a dosage to have much of an effect and excess AL can be toxic to many plants.

Much of the east coast has relatively acidic soils already so I wouldn't bother adding any AS unless a soil test shows pH higher than 6.5. And an application now will not affect this year's flower coloring anyway.

Check with your nearest plant nursery's master gardener and ask him/her if the soil in your area is acidic, neutral or alkaline. Or get a soil test kit from a plant nursery (not terribly accurate but, eh, close enough to answer "is it acidic") or better yet, make a full fledged soil test via a plant nursery or agriculture extension service/university.

Western PA is sometimes terribly acidic to the point that lime is recommended for certain crops & plants due to the very high acidity.

As a result, you may not need to do anything to get blues, except just plant the shrub. The shrub is now blooming pink because the potting soil was prepared by the wholesaler to produce pinks... instead of purples or blues. If it turns out that your soil is acidic, any future blooms will slowly migrate towards shades of blues if your soil is already acidic.

An imperfect, unscientific way of checking for acidity may be to drive around your neighborhood and notice if the majority of the colored mopheads that you see have shades of blue, purple or pink. Hee, hee, hee.

PS - by the way, the shrub in the picture looks very happy!

Professional soil test revealed soil in that location to be 6.4

Thanks gardengal!

Then you do not need much tweaking to the blooms' color. They may be a light to medium blue at most but wait until next Spring to find out. The exact shade of color is usually something that takes a couple of years, or so, to "stabilize"... due to having some roots in the potting soil and others in your garden soil.

Thanks!

Even in my highly acidic soil, I sometimes get pink blooms the first year, and once I had a plant with half blue and half pink blooms the second year. So you are correct that it needs to get roots into the surrounding soil before you know how it will color up in your garden.

Thanks...

I think they look great, Jim. I saw these in my Walmart a few days ago marked down to $7.50 for a nice sized plant. I may go get one as I've seen some nice posts about them on this forum.

Thanks 56steps!