Colour-banding is caused by lower bit-depths in images.
Images on Roblox seem to be limited to 8-Bits of colour, meaning an image can only contain 16 million colours. PNGs and TIFs are capable of 16-Bit colour, or about 281 trillion colours.
In 8-Bit images the only way to avoid visible banding is by using dithering. Blurring your image will have no effect on a regular 8-bit export.
The only issue I can foresee is that any compression on the image will interfere with the effect, this may be the issue you’re running into. Secondly, Roblox only accepts 1k images at the moment so uploading as 4k will have little effect.
Double-check that the image looks good before you upload it to roblox. If it’s bad, then the chances are you may be doing something wrong with the dithering.