A raw bullace will be sour, but a sloe will be lip-curlingly, mouth-dryingly tannic.
From the telegraph.co.uk
In addition, bullace trees have few, if any, prickles.
From the telegraph.co.uk
Bullace are small green or purple wild plums found in hedgerows throughout the southern half of the country.
From the telegraph.co.uk
We had a really early harvest of sloes and bullace in Manchester this year, so I have already had a fews glugs out of this year's batch.
From the guardian.co.uk
More examples
Small wild or half-domesticated Eurasian plum bearing small ovoid fruit in clusters
The damson or damson plum (Prunus domestica subsp. insititia, or sometimes Prunus insititia) is an edible drupaceous fruit, a subspecies of the plum tree. Sometimes called the Damask plum, damsons are commonly used in the preparation of jams and jellies. ...
A small tree or large shrub bearing black fruits 1-1.5in. long with comparatively large stones. Spreads by cuckers - shoots arising from underground from parts of the root system - and therefore often develops into dense stands. Frequently found near to former habitation; not much grown today. ...
A wild plum (Prunus insititia) larger than the sloe; there are two varieties, the black and the white.
A wild, purple plum. Brought to Europe from Jerusalem in 1198.