Took this pic from an occupied lot; the lot across the street doesn't match the slope of the road?? Or what is that BLUE?

Could you perhaps post a clearer/ larger image? That one is tiny!
Is the lot you are playing an apartment lot?

What does it look like from the neighborhood view, and when you are in that other lot? From those other views, is the road straight and aligned with the lot? Or does it still slope down there, but the lot terrain slopes down to match it? It's hard to say exactly what's going on without that knowledge, but I can think of a couple possible solutions worth trying:

1. Move the problematic lot out of the way, use the modifyneighborhoodterrain on cheat to make that part of the road level with the rest, and then place the lot back where it was.

2. Use the Lot Adjuster to make the lot edges conform to the slope of the neighborhood terrain. That might not be possible with the way objects are currently set up near the edge of the lot, though (is that turquoise bit in the top left a pool?) so unless you're okay changing/removing that stuff, this might not be a viable option, in which case you'd want to go with option 1.

Again, it's hard to determine what the issue actually is without more information. It shouldn't be possible for the lot to have a level terrain elevation while the road and neighborhood terrain directly beneath it does not, so either something funky is going on with the lot elevation, or this is an odd visual bug with the lot impostor that does not accurately reflect the actual state of the terrain.

(And to answer your question about the blue void, yes, that's the skybox, which is visible when you're seeing through a gap in the ground geometry like this.)

I THINK that can happen when you take holes that Sims dig to find treasure with moveobjects, it might not be that it doesn't match the slope. I was messing with moveobjects once and grabbed one accidentally, and apparently, undo doesn't work to get rid of that and you have to leave without saving.

Have you messed with the Lot Adjuster? I've never used it but by seeing other people's uses, it could be by that.

To all: it's not an apartment. There was no digging. From hood you can't see the blue, just a sharp dip in the road. And I wish I could bother to learn how to post bigger pictures. I think moving the other lot and trying to smooth out that dip in the road would work

Looks like terrain tearing, happens when lots that don't have flat edges are placed. And ofc I can't find relevant info on google.

This has happened to me with non-apartment lots and without messing with digging. I agree with Acrylonitrile and HugeLunatic, changing the lot's terrain or the surrounding area in the hood might help.

I've had terrain tearing, it's not uncommon if you have lots with odd elevations up or down.In this case it looks to be because of the elevation of the road. I would move the lot, use the hood cheat for terrain changing and bring that road surface up a little.