Why the fan leaves are getting black veins...........

IMO - a couple potential reasons dude. First - this is def. genetic and the plants look sweet👍. Very nicely done. That said, you’re coming into the final weeks so it’s not surprising that she’s starting to show off some colours. If ur on the flush yet, then the added colour is nutes in the leaves being consumed by the plant as well. The other thing of interest that has me wondering is how accurate is your temp. Variance indoors? A 4 degree diff. Day vs. Night isn’t much but temps do factor in significantly with this kinda colour change. I now do this purposefully in my space and can vary the temps from 26 to 17 throughout the cycle day/night. Even more if a vary the extraction fan speed. If you have the option then you can get some awesome results👍. Either way, nothing to be concerned about at all. These colours would have me giddy bro!👌

Thats a genetic trait , I wouldnt say thats black , its a very very very dark purple . I've seen these plants in real life from this company and they alllllll have that trait the purpling down the leaf until you see it like at your flower sites ! Nothing wrong here that I can tell just happy healthy plants ! Feel free to check out other grows on this strain you will see that this trait carrys on strongly :D I hope this information helps you on your quest to understanding whats going on here and that you have nothing to worry about :) -Happy Growing!