I generally oppose secession in democracies since it usually is very costly for all, and these groups usually have ways to have their interests addressed while staying put (yes, Quebec, I am speaking of you).
So, what do I think about the two hypothesized secessionist efforts: Scotland and California/Blue America?
I opposed Scottish separatism the last time because I didn't think the benefits were good enough, and the Scots seemed to have enough sway to get what they needed while staying in. Um, now? Oh my. I think that Brexit essentially represents the English reneging on the deal, that it is being yanked out of the EU. Sure, it may take time and heaps of paperwork to rejoin, but I could see why leaving the UK as it tries to destroy itself might be a good move. So, I am no longer opposed.
California/Blue America? In short: FFS. The plight of Blue America is, if the institutions are not destroyed utterly, temporary. Opting out because of a lost election is very much antithetical to democracy. If Trump utterly subverts the system and the US is no longer a democracy, then one can speak of separatism. But Blue America needs to stay put to rescue the rest of America from Trumpism. Leaving would consign all of the nonTrump people of remnant America to perpetual awfulness. Stay and fight, damn it!
Of course, the only folks who are serious about California secession are Russians, so maybe any fellow travelers/useful idiots might want to think about why that might be the case. Let's focus on resisting Trump, not surrendering to him, shall we?
--Sincerely, Cranky Steve