The blackberry drink comes from Ambrose Heath's 'Good Drinks' and because it's a Sunday I thought I'd find something non alcoholic. It's in the cold drinks section so not even a mocktail (though Heath doesn't use that term. His drinks are simply hard or soft.) however there's nothing to stop you adding alcohol, or using blackberry whisky instead of blackberry juice if you're so inclined.
Mavis Doriel Hay only wrote 3 books (all back in print as part of the Crime Classics series, the other 2 are 'Murder Underground', and 'The Santa Klaus Murder'), I found 'Death on the Cherwell' particularly interesting because of its portrait of 1930's Oxford student life. It was published in the same year as Dorothy L. Sayers 'Gaudy Night' and shares some themes with it. 'Death on the Cherwell' has its own charm though, as a period piece it lacks the troubling allusions to eugenics (or the hero worship of Lord Peter), for the current reader it remains more than a pleasant bit of nostalgia thanks to Hay's affectionate portrait of first year students. (I wrote a little bit more about it Here).
At 18 the would be detectives are caught between child and adulthood, still young enough to congregate on a boathouse roof to form a secret society dedicated to cursing the bursar, but not for much longer. Still young enough to enjoy going out in search of blackberries too I should think, although that's a pleasure that far outlives childhood.
This years blackberries are nicely ripe now so this is the perfect time to try this drink. You will want a quarter of a wineglass full (it would be a small wine glass and that's as specific as I can be), a teaspoonful of lemon juice, and a tablespoonful (or to taste) of fine sugar. Put in a large tumbler With plenty of ice, shake well, and top up with soda water.
There's a good recipe for blackberry whisky Here, which is entirely worth making. It really is better if you can leave it for a couple of years to mature (stick it at the back of a wardrobe and forget about it for a while, it's easier than you think) in which time it will become far more than the sum of its parts.