Those plates pictured above would have been among the first issued after the changeover in late 1969. Theoretically, the last pair of black plates isssued issued was ZZZ999, and the first blue plate number was 000AAA. Some early 1970 model year vehicles ended up with black plates, and some 1969s sold late in the year got the blue ones. Keep in mind that DMV offices in different regions would use up their batches of plates at different rates.
For some reason, commercial plates (issued to actual commercial vehicles as well as pickup trucks) didn't start getting blue plates until 1972. (Those plates are mostly numbers with one letter.) I've seen 1971 El Caminos with original black commercial-series plates on them.
The 7-digit blue plates began in 1980 when the DMV ran out of numbers again, starting with (in theory) 1AAA000. Blue plates coexisted with the reflectorized white ones (introduced in late 1982) until about 1987. After that, it was white only except for vanity plates, which for many years were still available in blue.
As to why a '66 Bronco ended up with blue plates, it could have been any of a number of reasons: Originally being sold out of state; originally sold in California but leaving the state and then coming back; one of the original plates was damaged or stolen; the vehicle was in a dealer or manufacturer demo fleet and not titled until several years later (unlikely but possible); or, somebody just liked the way the blue plates looked and traded up to them.
Yes, that last scenario did happen from time to time. Once newer cars started getting blue plates, the black ones looked "old" by comparison. I remember an article from Motor Trend in the mid 1970s where they compared the cost to refurbish an older car with buying a new one. The car in their project was a late '60s Dodge Coronet, and one of their finishing touches was to "replace those dated-looking black plates with new yellow-on-blue ones that modern cas have." Of course in hindsight that was a stupid move in the world of classic cars, but they were pointing out the relatively small cost of trading for new plates (probably $5-10 back then) to help make the car look newer.
I've never heard of anyone being able to get vintage blue plates issued under the YOM program. Yes, somewhere, somehow, some DMV clerk did it, but for the most part the YOM program cuts off after the black plates' orginal issue years.