
Crème de Cassis is a sweet, dark red liqueur made by macerating blackcurrants in alcohol and sweetening the result.
If you only need sweetness and color, a dark fruit liqueur (like crème de mûre) plus a touch of sugar syrup gets close. For the actual blackcurrant character, nothing substitutes — use another crème de cassis.
Blackberry instead of blackcurrant, in the same dark-berry register and at the same sweetness, and one for one by volume; the drink loses the blackcurrant tartness.
Blackcurrant syrup
Flavor without the alcohol, and sweeter. Put the alcohol back with a neutral spirit — about a quarter of the liqueur’s measure — and pour roughly half as much syrup as there was liqueur.
Sugar keeps it from spoiling but does nothing to stop it oxidizing, and cassis oxidizes visibly: over a year an open bottle darkens from bright purple-red toward brown and the fresh blackcurrant gives way to something closer to jam. Keep it in the fridge once opened and buy the size the bar turns over.

Tequila Sunrise
Tequila reposado

Kir Royale
Dry sparkling wine, Crème de Cassis

Russian Spring Punch
Dry sparkling wine

Stars and Stripes Shot
Crème de Cassis, Maraschino liqueur, Herbal liqueur (e.g., Green Chartreuse)

Alaska No. 2
London dry gin

El Diablo
Tequila reposado, Ginger beer

Original Tequila Sunrise
Tequila blanco

Arnaud
London dry gin, Dry vermouth, Crème de Cassis

Black Forest Gateau
Vanilla vodka

Honey Cobbler
Blended Scotch whisky

Liquorice Shot
Vodka, Sambuca

Parisian Cocktail
Dry vermouth, London dry gin

Kir – IBA
White wine

Russian Spring Punch – IBA
Dry sparkling wine, Vodka