So what am I missing here, how do I create a "4:3 from 16:9" modeline correctly? Thanks in advance!
Honestly man, some of the language used in this thread is a bit nonsensical.
854x480 is a 16:9 resolution, assuming square pixels (which you should always assume square pixels unless you know the game doesn't use square pixels, as is the case with NES,SNES,Genesis, etc).
So the only way to make it display 16:9 is to physically squeeze your CRT's raster. This can be done in 2 ways: if you're lucky, your TV has a "16:9" or "widescreen" mode that does this. Or, if you're unlucky, you just have to learn how to get in and out of your service menu quickly to reduce V-size.
In this world of playing modern pixel art games on CRT, you have to be cognizant of two things: the game's native pixel resolution, and the aspect ratio.
An easy example: Celeste. It's 320x180 16:9. Well that just so happens to be exactly the 16:9 area of a 4:3 320x240 image. So you simply have your desktop at 320x240 when you launch the game.
a bit more complex example: Blazing Chrome is 427x240 16:9. To play that, you have to have your desktop at 427x240 and then trigger your CRT's 16:9 mode.
This is really the only way to have games look like they're supposed to.
In your example of Blasphemous, that game is really too-high resolution for a CRT TV. It's 640x360p 16:9. You can technically play it just by running 640x480i, and you will essentially have an integer scaled 360i in the middle of that. But that's going to look odd, because A) no scanlines and B) because of interlacing you're only seeing half of the pixels in any given frame. So that game is best played on a PC CRT monitor, which can support 480p. Some HD CRT TV's could theoretically display it well too