Wondfos sometimes stay faint?

I had a faint bfp on 5dp5dt (frozen embryo). The same darkness every morning, but not getting darker. I'm now 7 days post transfer. My blood test is scheduled for the 10th. I heard Wondfos sometimes stay faint for a long time and then bam, one day, they can be dark? Just concerned that I might have a CP again. Wondering if anyone has heard this, experienced this? Need some hope.