Blue dyes always have an indented grey background. Below is a photo of an UNUSED generic blue dye brand. (Signature)

Now imagine when it is darkened by urine samples, it will appear much darker.

Furthermore they have horrible evaporation lines. If a line doesn't show up in 2 minutes, it's probably negative or too early. The box says up to 10 but in TTC I've found that after 3/4minutes it is an evap appearing as a false positive.

When they have lines, they are very obvious. I can post pregnant ones if you want to see.

I use them for checking hcg levels or if it is all the store has, but unless it is a line, in the middle, with edges (no blurred edges or off center) that shows up in one or two minutes, it is most likely negative.