Straif is the clearest instance in the row of a letter whose oldest recorded sense is not botanical at all, and whose modern meaning nevertheless works.
What the name points at
McManus connects straif to sulphur. The oldest of the kennings glosses it as the strongest of reddening, the most powerful of the agents that redden or dye.
Both readings point the same way, and neither points at a plant. They point at a process: the corrosive substance used in metalworking and dyeing, the thing that stains and etches and cannot be undone once applied.
The blackthorn
Draighean, the blackthorn, arrives in the later scholiast material, and the reasoning behind it is recoverable. Blackthorn in Irish tradition is the thorn that draws blood, the wood of the cudgel, the tree of ill luck, and its sloes yield a dark stain.
If a letter associated with reddening and corrosion must be given a tree, the blackthorn is well chosen. It is still a choice, made afterwards, by someone working to a scheme.
Why the modern reading survives the correction
Because it happens to descend from the older sense rather than the newer one. Straif is read now for necessary harm, for the thing that changes what it touches, for irreversibility, for consequences that cannot be withdrawn.
That is sulphur. It is not really blackthorn folklore at all, even though blackthorn is the name attached.
Which is worth noticing, because it cuts against the easy assumption that the medieval tree assignments corrupted everything downstream of them. Sometimes the older meaning came through anyway.