Germline Cancer Mutations

I need to know how are we getting germline mutations from blood? From where else can we get these germline mutations?
Germline cells are those that form gametes, right? So how blood has this germline DNA?

This a common point of confusion that comes down to terminology reuse.

• Your somatic mutations are those acquired after conception (e.g. present in your skin from UV damage).
• Your germline mutations are those present at conception (generally it can be assumed that they would be shared with your parents). The
  term “germline mutations” is somewhat unrelated to your actual germline.

Your germline is the cells that give rise to your gametes. In this case, these cells would passing on some fraction of your germline mutations (but also any somatic mutations that also occurred within your germline cells).

As to your question: The germline mutations that they detect in your blood are essentially mutations that should be present in every cell of your body (obviously mutations can revert etc… but you get the picture).