First Harvest, question on strain

So I just harvested about 20% of my first grow and started to get curious about what strain I may have (I obtained the seed from a dealers collection of various strains).
I noticed the leaves are turning a deep purple/almost black. Is this indicative of the strain (black widow?) or is it a nutrient issue that is causing the interesting color?

Thanks!

BTW grown indoors in soil with tierra a and b

Additional pics. With Halloween coming up I am really digging the black.