I'd say 1992 for CA, WA, OR, NV, CO, NY, NJ, PA, NH, ME, MD, DE, FL, IL, MI, WI, NM
1992 was a very big year since all those states have been solid blue or swing in nearly every election since then.
well let me review those states
CA - it already had a D+PVI in 84 and 88 but I would agree with you that that's when it started to become out of reach for a republican
WA - disagree. It voted for Dukakis in 88
OR - see WA
NV - mostly agree. Though I could argue the blue trend didn't become apparent until 08
CO - disagree. The state more than anything else tends to bounce around in terms of PVI. It actually went to Dole in 96 and voted for Bush by a comfortable margin in 2000
NY - it went for Dukakis in 88. I really see the blue trend in NY occurring in the late 50s. After 1956, the only time New York would go R is in landslide years like 72 or 84 or a split field in 1980
NJ - agreed
PA - disagree. The state has always been slightly dem leaning since the 50s. If you're talking about the Philly collar counties though, then I agree.
NH - agree
ME - the state had a dem trend as early as 1964, but I agree that this is when Maine consistently became a blue state
MD - agreed
DE - agreed
IL - agreed
MI - agreed
wI - disagree. the state voted for Dukakis in 88.
NM - see CO. the state's PVI has never been that consistent.