The cv's are doing all that intentionally so that the revelation doesn't happen soon, and the drama continues. I don't think the cv's introduced the whole blood group report thing not to utilise it, and they definitely haven't forgotten it. Remember the divorce papers that avni ordered during the track where avni-ali were accused? The divorce papers were used much later in time that even we almost forgot about them, and they were sent to the khanna's in avni's absence but neela was thankfully there to convince them that avni hadn't sent the papers by pointing out the date on the divorce papers being ordered by avni from a long time back. It was amol who broke into neela's locker or something from her house and had sent them to khanna's.
So when the cv's never forgot about the divorce papers, why would they forget such important reports that could be a huge clue to mishti's paternity? Why do I get the feeling the blood reports will only come forth to the family when avni leaves neil? I actually won't mind this considering that rat will be in an even bigger mess if avni leaves neil too because of her lie.