Prop 187 did not turn California blue, its effect is really very marginal compared to the status it has assumed. The key point of departure is 1992, two years before Prop 187 was even voted on, when Clinton won California in a rout. Prior to that point, California had been a very mildly Republican-leaning state in its popular vote but strongly Republican in outcome; the last Democrat to win it in a presidential contest before Clinton had been LBJ. Clinton winning it was a massive break with the past. George Bush went from winning it in 1988, to losing it by over thirteen points in 1992. 1992 is the sort of outcome we are familiar with today, and a thirteen point loss isn't just a result on the back of the nineties recession or whatever, it's a sign of a historic voting shift already underway.
What really turned California blue was the end of the Cold War and the coming of nineties politics; BRAC base closures, and defense industry cutbacks cost literally hundreds of thousands of jobs in metro LA. Millions of people dependent, either directly, or at one or two removes on a hard foreign policy had that gone. In the nineties you also have the fracturing of a lot of Cold War political paradigms, with Clinton, who was a solid fit for California voters on all the issues, and the GOP, shorn of its foreign policy focus and shifting southwards, going more extreme and emphatic on social issues and guns, which was not remotely a good fit for California. Extrapolate that process all the way to today.
If you want to keep California red, find some way of keeping the Cold War humming, and the political fractures the end of it produced kept bottled; that's a pretty deep POD though, depending on how resilient you reckon the Soviet Union was.