Blur is the loss of fine detail and texture - grass turning to a green smear, skin losing pores, brick losing mortar. It is the quiet member of the block-transform family, caused when the quantizer discards the high-frequency DCT information that carries fine detail, or when the video was downscaled and never recovered. It is also the flip side of deblocking: when a modern encoder runs out of bits, its in-loop deblocking filter smooths the unavoidable error into softness instead of seams, which is why heavily compressed modern streams look mushy where older video would have looked tiled - blocking is traded for blur. The hard part for both eye and metric is that blur is not always an artifact: a shallow depth of field is intentional softness, so the skill is telling a creative choice from a compression failure. SSIM and VMAF detect blur reasonably well because it is a structural, spatial change, but neither can tell intentional softness from a fault on its own.
