When purple anthocyanins turn black (or almost black)

[rbarata]

My Catt (former Laelia) tenebrosa developed a nice intense purple at the base of its new leaf.
Now it has been getting 5 hrs of winter sun, protected by a set of almost transparent curtains so I consider this normal.
But today, the purple area became suddenly dark, almost black. In any case I will move it so the it get less sun but I wonder if this is the beginning of a sunburn (or it's already a sun burn).

What do you think?

Before and after





Whole plant (almost)