Review:

As Robert Vermeire says in his 1922 Cocktails: How to Mix Them, "The Blackthorn is a very old cocktail, which is made in two different ways." The most popular way is with various combinations of sloe gin/dry gin and vermouth (see Blackthorn No. 1, 2, 4, 5 and Blackthorn English). This whiskey-based Blackthorn is the oddball in the long-running Blackthorn franchise. Dry and aromatic, it is whiskey's answer to gin's Dry Martini.

Variant:

There are numerous blackthorn recipes, but they fall into two camps: one based on Irish whisky with vermouth and absinthe and the other with gin/sloe gin and vermouth. The numbering of the various Blackthorn variations starts in the 1930 Swallows edition of Boothby's World's Drinks And How To Mix Them and continues in his posthumously published 1934 edition, where he adds the Blackthorn No. 5.
Blackthorn No. 1 - sloe gin, rosso vermouth, dry vermouth, aromatic bitters, orange bitters.
Blackthorn No. 2 - sloe gin, rosso vermouth, sugar syrup, lemon juice, aromatic bitters, orange bitters.
Blackthorn No. 3 (AKA Blackthorn Irish) - Irish whiskey, dry vermouth, absinthe, Bokers bitters.
Blackthorn No. 4 - gin, Dubonnet, kirshwasser, orange bitters.
Blackthorn No. 5 - gin, sloe gin, dry vermouth, lemon juice, sugar syrup, aromatic bitters.
Blackthorn (English) - gin, sloe gin, rosso vermouth, dry vermouth, orange bitters, aromatic bitters.
Blackthorn No.2 (Homeboy's) - Irish whiskey, port, sugar syrup, absinthe, aromatic bitters.

History:

The whiskey-based "Black Thorn" first appears in Harry Johnson's 1900 Bartender's Manual.

BLACK THORN.
(Use a large bar glass.)
3 or 4 dashes of absinthe;
3 or 4 dashes of bitters (Boker's genuine only);
½ wine glass of French Vermouth;
½ wine glass of Irish Whiskey;
Stir up well with a spoon, strain into a medium-sized wine glass and serve.

Harry Johnson, Bartender's Manual, 1900

Blackthorn Cocktail.
Filla bar glass half full of broken ice and add:
3 dashes of Absinthe.
3 dashes of Angostura Bitters.
¼ gill of French Vermouth.
¼ gill of Irish Whiskey.
Stir up well with a spoon, strain into a cocktail glass, and squeeze lemon-peel on top.
(Recipe by Harry Johnson, of New Orleans.)

Robert Vermeire, Cocktails How to Mix Them, 1922

The sloe gin-based version of the Blackthorn appears in William Boothby's 1908 The World's Drinks And How To Mix Them but the whiskey-based Blackthorn doesn't feature until the 1930 Swallows edition of Boothby's book, appearing as number three out of the four numbered variations.

BLACKTHORN, NO. 3
Whiskey . . . . . . . . . ½ jigger
Fr. Vermouth . . . . . ½ jigger
Absinthe . . . . . . . . . 1 spoon
Bitters . . . . . . . . . . 3 drops
Stir well with ice, strain into chilled glass and serve.

William Boothby, The World's Drinks And How To Mix Them, 1930

The whiskey-based "Blackthorn Cocktail" also appears in Harry Craddock's 1930 The Savoy Cocktail Book. Interestingly, Craddock relegates the sloe gin version, which he calls "Blackthorn Cocktail (No. 2)", to the "Additional Cocktails" section at the back of his book.

BLACKTHORN COCKTAIL.
3 Dashes Angostura Bitters.
3 Dashes Absinthe.
½ Irish Whisky.
½ French Vermouth.
Shake well and strain into cocktail glass.

Harry Craddock, The Savoy Cocktail Book, 1930

Nutrition:

One serving of Blackthorn No. 3 contains 155 calories