Apolaki, I believe that the black you can see is the steel core that has been hardened. The small specks in the body of the blade could be where the outside layers have worn, or, as Jean has suggested, maybe some steel is in the mix of the outside layers, but the big black areas at the point & edges are steel core.
As to cleaning with vinegar I am not nearly as scientific as either Jean or Marco. I buy ordinary white household vinegar, I have not the slightest idea of its acidity, I never dilute it. I simply wash the blade with detergent to get rid of any grease, and soak it in the vinegar. I inspect it once or twice a day and brush it as well as pick off the rust with a sharp tool, then I put it back into the vinegar. It might take me a week or more before I'm satisfied that it is clean.
Jean has my utmost admiration if he can get a truly dirty blade clean in 24 hours with only 2 brushings. Incredible! You're a better man I am Jean.
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