Walnut Sprouts are Black. Why?

I am trying my hand at something new: growing walnut trees from nuts.
Last Autumn, I put several freshly-harvested walnuts in a pot and covered it with dirt to go through the cold temperature stratification over the winter.
This Spring, the nuts were transferred into individual little pots.
Several of them have sprouted; they have shoots poking above the ground, but these shoots are black (see attached photo.)
They look like they have been burnt or something.

Any idea why this happened?
If it was something I did, I'd like to know, to avoid it for the future. I'd like to successfully grow happy walnut trees.

Probably froze. Mine look similar, temps don’t need to be freezing if skies are clear due to radiative cooling.

Tj Jefferson wrote:Probably froze. Mine look similar, temps don’t need to be freezing if skies are clear due to radiative cooling.

I concur.

Tj Jefferson wrote:Probably froze. Mine look similar, temps don’t need to be freezing if skies are clear due to radiative cooling.

Hmmnn... that is not good.
Shortly after I transferred them, nightly temps went down to about plus 2 degrees Celcius according to local weather reports. Perhaps in my back yard, temps, indeed, dipped to freezing. But the plants hadn't poked above the soil surface yet. Wouldn't they have had some protection since they were still under ground?

In any case, are these trees now effectively dead?

They will generally have some survival buds and should one back

yup. there's a lot of energy in a walnut. they'll probably keep pushing.