3rd edition would be the time when oldhammer coexisted with early newhammer?

This is why I subscribe to "Middlehammer" to reference 3rd edition. It's something of a unique phase where the influences of Oldhammer had been solidified into what anybody now would recognize as 40k, but it still had a lot of the grit that was part of the territory of old fashioned tabletop gaming and alternative scifi of the 1990s. 3rd edition introduced a lot of models that would go on to have very long lifespans as the kind of default.

I’d also guess it’s the release date which matters?

As a general guideline sure, but I am of the opinion that "Oldhammer" is a very malleable term. Some people lock in hard dates and model ranges, but I consider it to be more of a word describing a genre.

If we expand it to "Oldhammer vibes" there is modern stuff like Copplestone Castings, EM4, and numerous small 3D print makers who specialize in keeping that aesthetic of sculpting alive.

Eldar until very recently had to deal with old metal minis as their defaults and some tanks did not change in design since their release in the previous century. Are these oldhammer too?

I would consider metal minis from 2e as no brainer Oldhammer. I would feel comfortable considering metal 3e characters. With vehicles, the stiff metal walkers I would, but the plastic kits like the Falcon introduced in 3e has all the hallmarks of a more modern kind of kit.

have you seen any good mimicry of Blanche’s miniature painting?

Tons. Just do an image search for "Blanchitsu" and you'll see many great results. Of course the better results rise to the top and are going to be over represented on an image search. When I mentioned bad Blanchitsu I was thinking of the people who paint the whole model brown and black and then dump a pot of brown wash on it. There are a lot of those people, but their projects don't rise to the top of search engines.