We always have a degree of lutenising hormone present but it spikes as you would know, roughly 12-36 hours before ovulation. When the line is as dark as or darker than the control line- that is when the spike / surge has begun. What you are describing therefore isn't signifying the surge. As it is darker than previously it COULD be a sign the surge is approaching (but not here) but it could also mean nothing. Some people do have a fade in fade out pattern of darkening then lightening lines pre and post the positive opk but others don't- the line may be darker one day lighter the next etc. keep testing!