If you're growing the boxwood as hedging, then presumably it will be a low, clipped hedge - left to grow naturally, these shrubs will reach between 2 and 4 feet, with a similar spread, so it depends how tall you let the boxwoods grow in terms of how much you will see of the hydrangeas.
The hydrangea you mention gets 3-4 feet in height, with a spread of 6 feet or more. Because hydrangeas don't like soil that dries out frequently or easily, raising the level at which you plant them isn't a good idea because the raised soil will dry out quicker than the surrounding soil. So as long as you keep the boxwoods short, optimally at 1 foot, 1.5 feet max, you will be able to see the hydrangea flowers.