What kind of lighting and distance? How old are the bulbs?
What part of the plant is affected? (lower fans, perhaps?)
How much molasses are you using?
What's your ingoing ph, and if you have it, the runoff ph?

Did you follow the FF schedule in regards to week(s) 7 and 8 (weeks 3 and 4 of flower)...specifically the addition of Grow Big? This gives a little bump of nitrogen and micro's, delaying the need to scavenge from the lower fans late in flower. My belief is that it helps maintain proper balance for photosynthesis, too. But I have no proof of this, just a casual observation in my growroom. Speaking of the feeding schedule...do you follow the FF feeding schedule?

Now...we could play this 'I'll ask a couple questions and wait for an answer to ask more follow-up questions'...or you could speed the process by copying and pasting the troubleshooting form here, and fill it out to best of your ability...thus giving us all of the background info all the same time.
Rusty Trichome Reviewed by Rusty Trichome on . Blackening Leaves, Small Brown Spots A picture speaks 1000 words. Pic1 Pic2 The leaves are darkening and there are small brown spots on the leaves. The plant is at 34 days of flower. There are two other plants right beside this plant, and they show no problems. Nice green healthy leaves. Anyone good enough to figure this out from just a couple pics? I'm growing in FFOF soil and using FF nutes and molasses. You can see more pictures in my grow log in my signature link. Rating: 5