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Compression artifacts are loss of detail. Applying any kind of blur filter to an image that had compression artifacts would result in more loss of detail (and applying it strongly enough to remove the artifacts would pretty much obliterate any recognizable details).

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Actually, while applying a blur does obviously eliminate detail, you can retain most of the important detail with a selective blur. Give it a try in gimp and see for yourself. The default selective gaussian settings work pretty well for de-artifacting, and the loss of detail arguably isn't much worse that that in webp compression. Some of the commercial de-artifacting tools use different techniques (I don't know what they are) that result in even less loss of detail.

Compression artifacts add noise via coefficient rounding, just like audio codecs add noise instead of "removing sound you can't hear". However, encoders with poor psy-optimization (i.e. not x264) usually choose blurry quantizations that don't add much noise but also remove the original detail.

Also, the deblocking filter in VP8/H264 is nothing but a selectively applied blur that hides previous compression steps. It works pretty well.

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