Okay, I see there are two areas to look at.
The blurriness is primarily coming from itch.io resizing your image to its default.
I’m not how you ended up 357x324 by doubling, so for an example, let’s say we have an 80x64 image. Scaling that canvas up 800% would results in a 640x512 pixels. That resizing puts us over the 630x500 pixels, close enough not to distort noticably at a glance.
512 / 640 = ~0.79 500 / 639 = 0.80
They’re similar.
324 / 357 = ~0.91
That image is not only too small, meaning it’ll get stretched out, causing blurriness, but the proportions are also off. The pixels become misshapened and blurry.
Super easy mistake to make.
The fix I know of is to change the canvas size, revise the pixel art as needed, then scale up with Sprite Size -or- Export Resize.