ilovebotany4life, if my memory serves me well, the direction of the purpling is the key.
The purpling occasioned by phosphorus deficiency starts at the youngest tissue, the very center of the seedling. The genetic purpling/blackening of the pepper plant leaf starts at the tip and progresses towards the leaf stalk. I have observed P deficiency in seedlings once only - when some legume seedlings were germinated in sterile sand. I suppose that the cotyledons contained least of this nutrient so its deficiency showed up first. My conclusion would be that if the germinating medium were devoid of nutrients and the purpling showed up really early, then there is a P deficiency. If otherwise, it is a varietal characteristic.

I entered a discussion about purple leaves in Capsicum last spring. Following is part of what I wrote. You can review the entire thread if you follow the link below:

The most common cause of purple leaves (very common at this time of year)[I wrote this last Apr] is a deficiency of phosphorus (P) because P is required to make ATP and ATP is needed to turn sugars to starch and to load sugar into phloem for transport. No P - no ATP - no move sugars/starches - anthocyanin (purple pigment) builds up - plant turns purple. The most frequent cause of a P deficiency is cold soils, which is why so many plants planted out too early turn purple.
A P deficiency is not the only suspect cause of purple leaves. Nearly any environmental condition that puts the brakes on growth and the accompanying use of sugars, but does not limit sugar production (photosynthesis) can cause anthocyanin buildup and purple leaves.

If only the outer edges of the leaves are purple, it may be a K or Mg deficiency. If the center of the leaves are also purple, it could be too much Ca in the soil or the result of too much water in the soil blocking uptake of P and Mg.

Do not apply a fertilizer with the middle number higher than either of the other two numbers (N & K) to container plants. There is never a need for that much P (relative to N) in container culture unless you're supplying only N in another form. Plants use about 6 times more N than P and the massive doses of P in 'bloom-booster fertilizers' cannot do any good.

Al

Here is a link that might be useful: I'll take you to the other thread, if you click me

Black leaves are one of the most attractive features of the Black Pearl.

I run my lights 16 hours, and I keep the lights about an inch from the leaves.

Josh