The dagger was called this because the handle was invariably made of bog oak which of course was jet black in appearance.  This was a suitable very hard wood useful for the purpose.  The sgian dubh was consistently carried in a place of concealment very often under his armpit.  This gives support to the view that "black" does not refer to the color of the handle but implies covert or concealed.

When the Highlander visited a house on his travels, after having deposited all his other weapons at the front door he did not divest himself of his concealed stabbing dagger since in these far off days it was unsafe to be ever totally unarmed. Not because he feared his host but because he feared intrusions from outside.  Accordingly, although retaining the dagger, he removed it from its place of concealment out of courtesy to his host and put it somewhere where his host could see it - most often in his stocking on the right hand side if he was right handed and on the left hand side if he was left handed.

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