"Smoked" Perfect Fits

Preface: working towards L1 don't have sponsor yet, mostly just reading the comp rules and IPG.

Locally a few players have begun using the "smoked" perfect fit sleeves backwards so that they obscure the cardface by darkening the front picture of the card.

Looking through the Tournament rules it mentions that when playing with altered cards they must be wholly recognizable, and not marked.

My question would be legality of intentionally trying to obfuscate boardstate. Does this fit proper and allowable gamesmanship or start to edge into unsportsmanlike conduct?

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Using translucent sleeves backwards to obscure the card face is not a new thing and as you suspect, it's not OK. Doing it specifically to make cards harder to discern veers close to Cheating, in fact. While the MTR doesn't specifically make note of this niche issue, the Sleeves section explicitly gives judges the authority to disallow any sleeves that might interfere with gameplay.