That is certainly mysterious.

I can't offer much other than a vague anecdote. Many years ago (well over 10, maybe closer to 15) I remember buying a hydrangea from Rarefind, on site, that was a certain color I remembered as white or very very light blue...I think it was 'Hanabi' that was supposed to be white. Now, trips to Rarefind back the good ole days were often a rush of mental plant energy overload. It is possible I was about to pick out one in bloom, and decided another looked better w/o blooms, and bought it, picking up a different variety. Who knows. At any rate, the next summer when that hydrangea bloomed, it was the deepest, darkest, cobalt blue imaginable. I was dumbfounded. Never solved that mystery. It's up there with the "larch seedling that mysteriously turned into a pine seedling" I posted about a few years ago too. It seems like if you do enough gardening in a life, you're just going to eventually have stories for your own personal "Horticultural X Files" LOL.

I am of course aware of hydrangeas changing form pink to blue as the soil goes from alkaline/calcareous to acid. I am 1) not aware that there's a white to blue transition option and 2) not aware of it happening in a single season, as seems to have happened to me & Heruga.

Well a larch to pine is certainly an anomaly.. at least this hydrangea is still the same genus lol. I'll look for that post as I am now interested. I too thought it may have something to do with the soil pH but you're right it can't all of a sudden happen like this in a single season. You can't see in the pic but there were tiny traces of light blue on some of the flowers. Really not sure what's going on.

Are we talking ES The Original? Or a variety that looks like ES The Original? There was a poster in the Hydrangea Forum years ago who once reported his/her Nikko Blue once developed white blooms unexpectedly at a measured pH of 7.0 but I have not seen a similar post for ES The Original (yet). That too was unusual but I have not seen more similar posts since. Immature blooms can be white, green or yellow temporarily. There may be a difficulty translocating nutrients to the blooms. Just being curious, what is the soil pH? If close to or above 7.0, acidify the soil to 4.5 to 5.5 and add aluminum to see how the next/second flush of blooms appear in the summer/fall. Be aware that for blues, you may require pH around 4.5 to 5.5 before the flower buds open. Cultivars that are members of the Endless Summer Collection should produce a second flush once the new 2024 stems attain sufficient height to trigger flower buds and immediate blooming. Also wondering, does this shrub resemble ES's Blushing Bride? Anything also unusual in a recent soil test?

Well I can add a data point: my Endless Summer is also white this year! My first thought was it may be a reversion on a time clock like the alberta spruce in a recent article here. But luis makes me think I my have altered the pH by adding my compost this spring, which I overdid with wood ashes..

I have another piece of intelligence. White Flower Farm says phosphorous ties up aluminum in soil so one must use low phosphorous fertlizer to have deep blue hydrangeas. I had a bag of super phosphate someone gave me that I spread around a lot of things this year…

If you want, a cheap soil test with a soil pH kit with numeric results may be worth using in a pinch.

Looked back at it today, it is starting to turn a very pale blue.

This is not entirely unheard of. ES The Original has generally rather pale blooms to begin with and it's pretty easy to turn up various illustrations of mostly white flowers doing an image search. I am not certain what the precise cause is......soil chemistry, winter conditions, sun vs shade, maturity of individual flowerheads.......who knows. The color does seem to deepen or intensify as the season moves on.