It is one of the many, many often similar mophead cultivars of big leaf hydrangea, Hydrangea macrophylla. When does it bloom, just in spring or on new growth later in the season as well? That may help narrow down possibilities.

edited to correct spelling for mophead.

I think Babs meant 'mophead' rather than Moorhead :-) Dang the autocorrect/spell check!!

It blooms in the spring on old growth. Flowers hold deep purple coloring (even deeper than pictured) for several months.