Burke's General Armory gives no such blazon for Darnel or Darnell,
but there are couple of families of Darrel or Darrell with crest
Out of a ducal coronet or, a Saracen's head couped at the shoulders proper, bearded sable, wreathed about the temples argent and azure, on the head a chapeau of the last, fretty of the third, tasselled gold, turned up ermine.
The word 'sharnue' would appear to be invented!
Certainly Complete Guide to Heraldry by Fox-Davies gives no clue as to what it might mean.