Butcher Block

Oak's high tannin content tends to react with any iron or iron compounds to produce the black staining.

Quite likely.

Something like a Quick-Drying Tongue/Tung Oil can be good - safe on food contact surfaces as well. (pure Tung oil is also ok, but needs *lots* of time between coats).

You might find it better to clean the oak with oxalic acid before scraping the surface. Even extensive black marking from iron will quickly vanish.

Paraffin Oil as a surface treatment has the advantage that if the surface becomes marked again it is easy to clean it.

I tried Oxalic acid a few times ... even followed recipe that used to be in FAQ using 'Bar Keepers Friend' failed to remove anything May just be me though.

Bleach works though

I've used Oxalic Acid many times to remove often heavy iron stains on oak and it has always worked. Mix a saturated solution of oxalic acid in water by adding acid crystals to water until no more will dissolve (about 150g in 1 litre of water at room temperature (25degC)).

(BarKeepers Friend contains only between 1 and 1o% Oxalic acid.)

Brush on liberally and leave for about half an hour. Repeat if necessary.

Oxalic acid is a wood bleach so will clean other stains from wood as well.

Available on ebay - eg