Logwood as I recall is from the Campeche tree. It contains Haemotoxilin a strong black organic dye that attaches naturally to iron. Thus it's use in microbiology. So yes if it's oxidized off then it leaves little black behind. I agree that the photos show great contrast. But the black oxide you have created is very unstable and will decay quickly (relative term) from the ferro ferric oxide to ferric oxide or rust. This does not happen from an appearance perspective with Haemotoxilin present because it remains carbon black in color regardless of the conversion of the oxide. I wrote this from memory so forgive me if I've mis-spoken.
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