Should I retest?

Can implantation bleeding ever be dark? Just curious. I was 9 days late for my period and only had bleeding for 1 day. I used one tampon, then just pantiliners for another day. Everything was gone by the second day but I did feel cramps the 1st day of bleeding. I have 2 first response tests, but I don't want to waste them. Should I retest or move on? my Walmart cheapies were negative as far as I could tell.