Black tips on vanda roots

This is the picture of Vanda tessellata with new roots in a vase ( reason why the picture is distortedly fuzzy).



Why there are black tips on the new roots? I didn't fertilize for a long-long while(shame), so it is not a burn from ferts.
My vandas do live on the windowsill and we got down to -30 C outside, therefore I am inclined to think that it is from cold. I moved them away from the window for now.
Are there any other possible reasons for such black tips?

Update: roots resumed their growth. I do think it is from cold air indeed.