I've read the entire thread...and have this to say:
One...the blue line is normally the control line. If it does not show up...it means the test is invalid...either out of date or in some other way not reliable.
The line which normally shows the presence of covid is the red line. If the red line shows up (with the blue line) the test is positive...no matter how light or dark red the line is.
Read the instructions more carefully...and take the test again if you still feel poorly.
My wife and I both felt like we had a cold...and took a covid test (she a day before I). Hers showed both the blue line (valid test) and the red line (covid present). I took the test the next day and I also tested positive. That was last Tuesday for me.
We both isolated completely...no going anywhere except for short walks in our own yard. No contact with anyone except ourselves.
She improved almost immediately...and retested on Saturday. And came back with a negative result.
I improved almost immediately also...and retested late yesterday. That test also was a positive. So today, I will contact my doctor to ask what to do next.