Pink test v. Blue test

AK
The other day I decided to take a blue dye test, I got what I thought was a FAINT plus sign.. I tested the next morning with FMU and pink dye and it was definitely negative. 
I am on cycle day 37, I've been regular for the past year (27-29 day cycles). I've only ever skipped AF when I've been pregnant. 
Can someone help me out and give me an idea as to why my tests are coming out negative?! Is it too early still? I'm at least 24-25 DPO

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