Blended textures is good and seamless in Cycles but have seams and new detales afterm baking to new texture on this mesh
is it my fault or texture blending is still experimental and useless for production?
image|690x379
Change flat to box mapping and adjust blender value
you don’t understand. i’m baking from box mapping to uv mapping
It should bake from box to uv as expected (tested just in case), so must be a problem with something else. We’d need more information about your process.
you can bake witout seams? show me please
i just bake from box mapping blended texture to uv texture. and have seams
i have this
hmmm, this could be a resolution problem maybe? You seem to downscale a lot…
downscaling or upscaling not matter
it seems Blender cannot into seamless bake from custom uv
I’ve made a few more bakes and I see seams now. But it would seem it’s not related to baking from blended texture at all, but rather has to do with uv seams in general.
So far I don’t see a way aside from setting high resolution (where the seam is still present, yes, but not as visible) 
useless. normal map with seams spoil everything.
If Blender cannot into seamless bake from custom uv - it’s useless for production