I really didn't understand what "germline dna" truly stands for. Everyone is saying that germline mutations can be identified with blood cells while somatic ones can only be identified from the tumor tissue where somatic mutation occurred in. I don't understand how we can see germline mutations from peripheral blood cells.
You can get germline mutations basically from every healthy tissue. The point with blood is that it is easily accessable via blood withdrawal, resulting in millions of cells from a single donation. You could also get it from pancreas, liver, lung or spleen, but that would require invasive techniques. It is simply a matter of convenience.