If you have blurred vision, visual acuity and retinoscopic view in a inproper fit contact lens will be distorted, but with blinking, vision improve immediately.

Minimal or excessive movement or even immobility of the lens.

When you see halos around lights, this is because the decreased oxygen into the eye tear film caused by tight contact lens. You need to pay close attention to halos,itching or feeling foreign body sensation. This may even lead to corneal hypoxia and other eye disorder or diseases. You optometrist may tell you a poor fit or extended wear of a contact lens will cause asymptomatic complication named corneal neovascularization.

Even your black contacts do not need prescription, you should still be looking for these to make sure you get a well-fitted contact lens:
Adequate lens diameter – full limbal coverage.
When you are in straight gaze, the lens shouldn’t move more than 1.0 mm with each blink.
when you look up, the lens should move no more than 0.7mm.
When you look straight, gently push the lower eye lid, the lens should be able to move without a problem