A reading of 7.0 is considered neutral; below that, it is considered acidic and that is what hydrangeas prefer. At that level, pink mopheads will start to turn shades of blue (the lower the number, the bluer the shade of blue) as soon as their root systems are able to absorb nutrients from your soil. Blue mopheads will adjust to the appropriate shade of blue for your soil. The best way to get an idea what this shade of blue will be is to drive around looking for other colored mopheads.

For the time being though, most of the nutrients will come from the potting mix since that is where most of the roots are now located. And the blooms will be whatever color they are now.

But as the roots grow into your garden soil, they will absorn more aluminum and gradually change colors. Being so late in the season, I would look for more developments in the Spring, color-wise. I would expect the gradual change to take a few years to completely "fixate" on the color that is appropriate for your soil pH. I would also expect to see multi-colored blooms for a few seasons.

Enjoy the show Miss Mary!

PS - At more acidic levels below 6.4, the blues may be more vibrant but levels that are too acidic (say, below 4.5) are close to the level where that can make it hard to absorb other minerals. Similar to the iron chlorosis problem of alkaline soils but this time it would be a phosphorus deficiency, or a nitrogen deficiency, etc. So be careful if you amend your soil to make it even more acidic. Try staying above 5.0.