Shoes have insides as well as outsides so blood would have been visible/detected on the inside if EP had bothered to examine them----which they didn't as they weren't given an exhibit number for starters.
Why the need to " say nothing " when SJ took them away ?
I'm willing to bet the inner sole of flat, black canvas footwear would also be black. Blood can only be detected if there's blood there to detect. I can't say what was the need to "say nothing" but it crosses my mind that perhaps they should have been seen previously? I can see no valid reason why he should wish to draw AE's attention to them. However, what I AM willing to put money on is that IF those shoes were blood-soaked, the blood was dry, and if it was dry, it would have been flaking off in a fine dust, from the canvas. I doubt AE would have missed it.