The Sub-commentary states: Dwelling at the extreme of Great Exceeding represents surpassing the limits to the utmost. Treading into danger and difficulty to the point of being submerged over the head signifies wading deeply into peril.
Once the head is submerged, it is 'misfortune.'
As for 'no blame,' the reason one wades into peril to the point of being submerged and meeting misfortune is that the original intention was to save the times and rescue from distress; the intention was good even though the outcome was disastrous, so no blame or censure can be attached.
This is like Longfang and Bigan, who sorrowed over the perils and chaos of their times, feared not execution, and offered direct, profound remonstrance that offended their unprincipled ruler, ultimately meeting destruction.
Their intentions were good, though their mission failed; what blame or censure could there be? This too is the image of 'wading beyond one's depth until the head is submerged: misfortune, yet no blame.'
Thus the Image says 'cannot be blamed,' meaning it does not violate moral principle.