
Crème de mûre is a sweet blackberry liqueur made by macerating blackberries in spirit and sweetening heavily.
Everything in the family is sweet; how sweet is the variable, and the EU floor for the sweetest of them is nearly double the rest. Taste before you cut the syrup, and expect the fruit to change from blackberry to whatever the bottle actually holds.
Crème de cassis
Blackcurrant, and by law far sweeter: 400 grams of sugar a liter against 250. Substituting by volume puts noticeably more sugar in the drink.
Blackberry brandy and eau-de-vie
Dry, distilled and much stronger, and it brings the fruit without the sugar, so the recipe has to supply the sweetness itself.
It will not spoil, and this is the style that fades soonest: the fruit aroma is the volatile part and the color goes with it. Closed, cool and out of the light. A long-open bottle is a weaker version of itself, not a bad one.