Really common with clear blue. Exactly why you should throw them away and not look again after the testing window
Blue dyes are horrible for giving evap lines! Definitely wouldn’t take that as a positive unfortunately. Try a pink dye and checking within the time frame the test recommends x
It’s pretty common. As the test dries a line becomes visible. That doesn’t make them positive, and that’s why directions give you a time limit in which to read the tests