Does anyone know how remontancy was bred into hydrangeas?

There's not a simple answer to this question :-)) It is generally accepted that Hydrangea macrophylla produces flower buds in fall in reponse to decreasing
photoperiods and cooler temperatures.Typically, these are the only flower buds produced and if removed by pruning or damaged by winter cold, the flowering would be theoretically be lost for the season. However, studies conducted at Center for Applied Nursery Research in conjunction with the University of Georgia in the 90's determined that this is not necessarily the case, as flowering was observed on a number of cultvars originating from the current season's growth (as opposed to only 'old growth', as was previously believed). Remontancy was tested with these various cultivars under specific greenhouse conditions and the best performers were then selected for breeding purposes for the current crop of "rebloomers".

I find the term "rebloomers" confusing and almost misleading. The plants don't exactly rebloom - they bloom from undamaged buds formed the previous season as well as from buds formed during the current growth cycle on 'new' wood. Old growth will never produce more flowers than buds that were present in fall and there is a limit, based on photoperiod, as to how much bud set and flowering new growth can produce. The bloom period is just longer, all things being equal :-))

Living in a mild climate like CA, I'm not sure how much weight you need to assign to specifically remontant or reblooming clutivars - you have an extremly long bloom cycle anyway, given your climate. My sister's hydrangeas a little further south of you in OC were in bloom from April/May until October, sometimes later. The reblooming forms were developed primarily to benefit colder weather gardeners, who were at great risk of losing flowering potential due to cold winters. Personally, even in my climate, I don't find reblooming hydrangeas bloom any sooner or longer than the supposed non-reblooming forms.

To my knowledge, there are no pure white reblooming lacecaps, but there are several mopheads that fit the bill. The Forever and Ever series offers 'Peace' and 'White Out' and then there is 'Blushing Bride' - here, the only blush or pink tones come as the flowers age, which is a very common phenomenon with white flowers of many different plants.

Or you can experiment with several of the older cultivars that have demonstrated obvious remontant tendencies. These include 'Mme. Emile Mouillere', 'Lanarth White' and 'White Wave'.

And no, there is no amendment to make a white flowering hydrangea whiter. The pink coloring is due to aging of the flower and the change in pigments, much like fall leaf color.

White Out and Peace are my usually recommended choices. Based on your comment of BB being too blush for you location, I should also mention that some white blooms will -at times- develop a yellow tint in the center when they originally open. It is temporary and goes away as the bloom matures.

Mme. Emile Mouillere has not shown remontant tendecies in my garden in Z8. It is however, an extremely reliable bloomer. This year, it had some bloomage while most of my mopheads had none. Not as much blooms as in previous years, mind you but, still quite surprising compared to the majority that is all green this year. MEM's blooms last here until December but that varies based on how mild/strong each winter turns out to be.

Lanarth White did not show remontant tendencies for me either (when I had one in years past) but, let me tell you, it won the "best white lacecap" prize amongst all the lacecaps I have ever had. If you ever want a lacecap, I recommend it.

Do remember that as the blooms age, as prev mentioned, they will turn other colors so they will not remain white forever until they fall. I have some whites still in MEM but most are now turning greenish/pinkish.

If interested so much in whites, consider paniculatas too. They bloom white originally too but, the bloom is panicle shaped (not a roundish mophead). They produce flower buds in Spring and you get blooms around June-July-August depending on the variety.

Enjoy shopping for white ones but do note that at this time of the season, there may be few out there locally and in the Internet (you may see the dreaded 'out of stock' sign) but later in Spring 2015, the supply should ratchet up.

Luis