First, here are just a few of the websites I looked at:
The following Blackthorn description is taken from http://www.wicca.com/celtic/celtic/
Blackthorn is a winter tree. Its white flowers are seen even before the leaves in the spring. It is black barked with vicious thorns and grows in dense thickets. The wood is used in the cudgel shillelagh and Blasting Stick. Its thorns are used to pierce waxen images. Blackthorn indicates strong action of fate or outside influences that must be obeyed.
As soon as I read this description of Blackthorn, I thought Severus Snape. Imagine a tree with black bark and no leaves, only white flowers and thorns. One might think of Severus Snape’s attire and hair as being almost a dark bark, his pale skin, the white flowers (though comparing Severus Snape to a flower seems odd to me).
The Blackthorn’s fruit is called sloe. Immediately, the phrase “sloe-eyed” came to my mind. I’ve always thought that expression to mean someone with dilated or very dark eyes. Severus Snape most certainly has dark eyes.
It has thorns…dearest Severus definitely seems to have thorns of his own: his words.
…and doesn’t it just seem typical that it would be a winter tree? This also goes along with his winter birthday as well.
As far as the symbolism behind the wood…“Blackthorn indicates strong action of fate or outside influences that must be obeyed.” I think that speaks for itself.
http://www.symbolictrees.com uses the following symbolic words for the Blackthorn: