Can Kale be hot like horseradish?
There's a chance the extra sugar from the blueberries has helped bring the peppery taste from the kale out a bit more. Kale is known for its light peppery flavor much like watercress but not any where near the heat of horseradish. The addition of sugar, much like salt is great for intensifying flavors.

Rather than try and describe the taste of kale myself, I'll use the words of someone else instead.


  The flavor is peppery and assertive, and can have a pleasantly bitter edge.


Source: A Sweet and Savory Life &lt;- A good read if you are more interested in kale.

So, in reply to your question. Yes, Kale can taste hot like Horseradish.