Hello there,
You must be observant!(or I just don't watch the observation part well enough?)
This is Horesradish
http://www.dkimages.com/discover/previews/961/ 85025493.JPG
but I feel it might have been
scorzonera, also known as black salsify
http://www.patnsteph.net/blogpics/salsify.jpg
There is also a "white" salsify too
http://www.veggiegardeningtips.com/wp-content/ uploads/2007/08/salsify-oyster-plant-root.jpg
called "white" because when scraped and cooked it IS white,whereas the black is dark after cooking.
Yes,they do look the same! LOL
I have eaten them both,and neither has much taste,but they are pricey to buy(because they have a very short season) .