Is it possible to achieve blending so smooth that you can't see line or crossing between 2 colors?
If it is, how?

https://www.planetfigure.com/useralbums/peedees-stuff.758/view
Have a look at my gallery.
all painted in acrylics.
If you thin your paints at least 50% and apply in thin coats
there is little visible transition between colours.

Wet in wet painting (also known as wet blending) and glazing are the two best techniques for that.

However; nothing in life is actually that smooth so why. Look at the painting of people like Kirill Kaneave (most of the Young Miniatures box art for example) or Sang-Eon Lee (Life Miniatures box art). Neither of them paint completely smooth. Both use acrylics. Both have their own styles which often involve stippling to blend lines.

My painting (all acrylics) https://www.puttyandpaint.com/Forte

And others who are way better than I am but also do not paint smooth.
https://www.puttyandpaint.com/PepaSaavedra
https://www.puttyandpaint.com/rusto
https://www.puttyandpaint.com/BansheeArtStudio
https://www.puttyandpaint.com/Franciuus
https://www.puttyandpaint.com/Arnau_Lazaro
https://www.puttyandpaint.com/sergiocalvo
https://www.puttyandpaint.com/jarhead
https://www.puttyandpaint.com/Yellow_one
https://www.puttyandpaint.com/mmasclans

I'll second Peedee. I haven't quite mastered it yet, but this is the technique I'm practicing. Build up thin layers of color.

Prost!
Brad