The Sunset Western Garden Book states that hydrangeas like full sun but prefer "partial shade in [the] hottest climates."
I live in USDA Zone 9, Sunset Zone 17. My next-door neighbors, like I, have an east-facing house. They have a very large bush hydrangea in front of the house. It gets morning sun and afternoon shade. They've had to cut it back a couple of feet or so every couple of years b/c it gets so large.
Re. bloom color. Again from Sunset: "In some hydrangea varieties, blue or pink flower color is affected by soil pH--bluest color is produced in strongly acid soils (below pH 5.5), pink or red in neutral to alkaline soils (pH 7.0 and higher)."
So it's not a loss of nutrients, but an acid/alkaline soil balance, that affected the color of your blooms, daisee.
I'm trying to rescue my MIL's hydrangea. BIL had it set into the (very) shaded northwest corner of MIL's lot, three feet in from the corner of the lot--which is surrounded by solid 6-foot fence. The irrigation system has NOT worked, and on my March, April, and May monthly visits to MIL's house, I have watered the daylights out of this poor hydrangea b/c its dirt was dust.
Last Saturday, I found a bucket of rotting (and I mean anaerobically rotting) agapanthus roots/water/who knows what else and dumped them near the hydrangea for nutrients AND for disposal far away from everything else. (MIL doesn't have a compost bin--YET.) My hope is that these nutrients will help the hydrangea make it to my next visit on June 8. (I go down there about every 4 weeks.)
Good luck to everyone with hydrangeas! I absolutely do not have a suitable spot for one here, but I do like looking at my neighbor's plant and am good to go with rescuing MIL's plant.
El Cerrito, CA