Filter Mode is how a texture appears when scaled or skewed in the viewport. By default, textures are bilinearly filtered, which means the points between pixels will blur between each other when not perfectly aligned.
Below the Advanced dropdown of the Texture Importer, there is a Filter Mode setting, switching this to Point (no filter) will remove the blending of pixels, and colour will snap from one pixel to the next no matter how it is scaled.
Also consider using Unity's 2D Pixel Perfect package so the camera consistently aligns to a pixel grid relative to your sprites.
More robust filtering techniques that specialise in pixel art exist that allow for much more flexible and less aliased results:
Unity, huh, how?