Any changes to the root area in the last year, Stera. I.e., trenching, fence building, grade changes, paving, heavy equipment driving by, etc.?

Nothing Tug. (That was quick!)

Brown marks on the petals are usually a result of frost damage. I'd say it was the weather conditions this year - if you recall there was a question from Arbuthnot recently about the fact her M. stellata had open flowers near the bottom, with some unopened buds above, and possibly evidence of even less developed flower buds higher up,so it might just be the randomness of the weather.

It could well be that, as I just noticed the Azalea has a touch of the same -whew what a relief! I don't remember seeing a frost but the Magnolia (but not the Azalea) does get the morning sun and I'm not one of your early risers....
Well at least the unopened buds should be OK when they get around to doing their thing. (Yes I do remember Arbuthnot's question but hers didn't appear to have flower buds right at the top and mine has big ones almost ready to open - just thought it was unusual for them to open later.
I wouldn't have panicked it if had been any other plant.
And yes, the weather has certainly been unpredictable to say the least.

I agree, if you had a warm spell, followed by a frost, then it's most likely frost damage to the few pre-mature buds. The tree itself isn't damaged, you just missed out on a few blooms. Snip off the ugly brown blooms and you should still have plenty of buds left for a decent display. The same thing happened to my favorite rhododendron. It was trying to bloom in February but an "Alberta Clipper" sweeping across the plains turned purple into brown overnight.

Frost makes sense Paul but I was unaware that we'd had one. The flowers were in full bloom and beautiful.
This strange weather is confusing lots of plants.

It usually happens overnight.

Well yes - and there are usually traces of it under the north side of the hedge in the morning. I must have got up rather late that day...