Apparently, this is a little like slow gin.
I borrowed this recipe from the The River Cottage web site.
Ingredients:
2kg / 4lbs blackberries
225g / 8oz sugar (or less according to taste)
1 bottle whisky
Place the fruit, sugar and whisky in a large screw top or kilner style jar. Shake every few days until the sugar has dissolved.
Place in a dark cupboards for three months, turning slowly to mix once every two weeks or so.
Strain and bottle. The whisky will have turned a deep, dark purple. It can be drunk straight away, but the flavour will continue to improve for up to 2 years.