GC First try

Dark / metallic blue leaves. That's a classic phosphorous deficiency. Your plant is about to enter flowering and needs flowering nutes. You might also see irregular chlorosis with burnt edges and spots, which is potassium deficiency.

Cannabis potassium deficiencies can cause your cannabis leaves to turn white, yellow, brown or burnt looking, but the inside veins almost always stay green. Sometimes a Potassium deficiency is made worse by overwatering, as was the case with this plant. Sometimes a cannabis potassium deficiency (like all deficiencies) can be triggered by stressful conditions (for example overwatering, heat, transplant, etc) and may clear up on its own after the period of stress is over. If you only see one or two affected leaves near the bottom of the plant, and the problem isn’t spreading, I wouldn’t worry too much about it! In soil, potassium is best absorbed by the roots in the 6.0 – 7.0 pH range. If you suspect your growing cannabis plant has a potassium deficiency, flush your system with clean, pH’d water that contains a regular dose of cannabis-friendly nutrients. Old damaged growth will likely not recover. Watch plant over next few days to make sure that the problem stops spreading to new growth.

Hi @Xalium! That's a Potassium deficiency. The first symptoms of this deficiency can look like a nutrient burn, but it's not, a nutrient burn usually affect the newest leaves while a Potassium deficiency affects the oldest (lowest) ones... As she entered into the flowering stage, your plant is asking for more Potassium to properly develop her bud-sites and expand the flowers. I don't know much about the nutrients you're using but I would recommend to look for a good organic P+K booster. Hope this will help, let me know in the comments section if you need any further information, and happy growing! 👊