Is it permitted to use wood from used wine barrels (that were used for non-kosher wines) for baby blocks (there are dark wine stains still on the wood).
Can it be used for a food cutting board?
It is permitted to use the wood for making baby blocks.
It appears that there is no problem in using the wood even as a cutting board.
The Shulchan Aruch (Yoreh De’ah 137:4) writes that it is permitted to use a barrel that was previously used for non-Jewish wine for purposes of storing water, beer, and so on, provided that the barrel has been cleaned of residue liquid.
Although it is forbidden to use it for wine, it is permitted to use it for other liquids, because the taste is “pogem” them (see also Taz 7; Shach 15 writes that this should not be done on a regular basis; see Darchei Teshuva 11 who defers his stringency). This is very relevant for the question of whisky that is placed into wine barrels.
Thus, we see that the barrels are permitted in deriving benefit. Likewise, there won’t be a problem in use as a cutting board. It is clear that it is permitted to use the barrel even for sharp drinks (the Shulchan Aruch mentions them), and the beliah, as noted, isn’t a problem because of the pegam.