You did say
most of my 3 week old seedlings are yellowing
. If I had to guess I'd say that over watering was likely, either that or some kind of root rot. The only time I've seen black leaf tips are on failed clones. Overwatering either seedlings or clones will cause yellowing, wilting and ultimately dead tissue. I'd have to agree with Garden Knowm, Although I'm far from an expert.