dark discolouration developing in flowering wk 3

Black or blueish patches on leaves (specially bottom ones first) are a first symptom of phosphorous deficiency, which is very common in the flowering stage, it means you need to up the flower nutes. Also I have to mention that it can also happen due ph been out of range and since you don't post pH values, I have to assume that you are not measuring and adjusting it, meaning it could be a reason for the issue you are having. Whatever the cause, the patches will start to look necrotic and if not solved, the issue will continue, severely affected leaves will develop dark metallic purple color patches, will also curl, wither and drop, as the plant takes the phosphorous out of the bottom leaves to try to power the bud formation.