Grid's been filled and I have had 4 obvious candidates for the 3x3s for hours now. But however many times I come back I can't figure out the SIs.
What's a 3x3 'of four corner letters'??
Surely if each 3x3 only has one black square, that has to be the middle square (ie it must, by default, have letters in the corners)?
Harrumph...