Is your feature request related to a problem? Please describe.
This is somewhat of an inconsistency.

Describe the solution you'd like
There should be an option somewhere in Video Settings that disables the interpolation of biome colors, much like vanilla's option to turn off smooth lighting.

Describe alternatives you've considered
N/A

Additional context

I'm not sure I understand this feature request - isn't there already a slider for biome blending that lets you turn it off? Or are you asking for an option to keep biome blending on, but make it look "blocky" like non-smooth lighting is?

The latter is what I'm looking for here.

This doesn't seem like something in Sodium's scope as vanilla doesn't have that option - maybe ask the dev of Sodium Extra?
I'll leave this open for the maintainers to make the final decision on though.

Setting Quality > Biome Blend to 0 is "blocky" like non-smooth lighting.

I think they still want biome blend, just not blended over the block faces. So a gradual transition over multiple blocks, but no blending within the blocks.

I assume they want something like this

The above image is exactly what I'm requesting here, yes.

Another effect of the intra-block biome interpolation is that it seems to make the effects of the Ambient Environment mod (which varies colors within biomes) a bit harder to see, since the colors don't pop against each other quite as much.

"Vanilla", biome blending off:

"Vanilla", one block of biome blending:

Ambient Environment, biome blending off (not a recommended setting, but shown here for comparison):

Ambient Environment, one block of biome blending:

Ambient Environment, one block of biome blending plus Sodium's added smoothing: