The blooms mature and get old, causing a progression of color changes that ends when they turn brownish. All hydrangeas have blooms that "stay" a given color for several weeks (2-4) before changing colors. The color progression differs by hydrangea variety but it should be the same year to year. Sort of.

The potting soil in which your plant came produced the original color bloom that you saw: blue. But now that the plant is growing outside of the pot, the alkalinity will eventually switch the bloom color's next year a tad towards pink-ish. It all depends on how fast the roots grow out of the area containing the original potting soil. Expect the original blooms to turn either pink or light blue next year. Amend with aluminum sulfate if you want to keep the blues.

Also known as spent blooms!

Yet another sticky topic this forum seems to need. Yeesh!