I did everything you listed, and the issue is still there.
Try using PointLight, for this problem.
That already seems like an improvement if you ask me, if you keep continuing this cycle and extruding it by its sides then I personally believe the dark blue hue should disappear after some adjustments.
I will try that out when I get back on Studio. 
Scale it all the walls backwards, so they appear the same and are huge, blocking out a lot of light. Scale the roof in every direction but down, so that the build appears to be a 3D rectangle from the outside.
This is usually what I do.
Unions are a very bad practice, I do not recommend that the OP does this.
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No worries. I donât want it corrupting and disappearing like your head. 
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Enable the shadows setting in the pointlight or whatever light youâre using. If it looks bad, duplicate it a few times.
Hopefully Iâm not replying to a solved post
Edit
I didnât read this correctly, though you could use an atmosphere to control this
I was trying to fix this for my game and I found that changing the value of the Ambience worked
im here to save the day, i was recently was working on a cave map and found turning down enviormentalspecular and diffuse to 0 or close to in the lighting tab âshadowmapâ of course to work well.
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I actually tested this out and it proved good results!
With diffuse and environmental specular:
Without:
Since the apartment was made on the new baseplate, certain lighting settings (including the diffuse and environmental specular) are automatically enabled.
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All these people helping me and I canât use Roblox Studio because of exams. 
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