Pure Yin constellation means that the heavenly stems and earthly branches attribute in your constellation is all negative.
For example, heavenly stems: A, B, C, D ... .
A is Yang wood, B is Yin wood, C is Yang fire and D is Yin fire. etc
So is the earthly branch.
There are many sayings of eight-character pure yin. There is a saying in marriage that a person with eight-character pure yin matches a person with eight-character pure yin.
I hope it will be helpful to you and I hope it will be adopted.