Blumensaat's line is a linear shadow on lateral radiographs of the knee that corresponds to the roof of the intercondylar notch between the two rounded condyles of the femur.
The roof of the femoral notch
These two X-ray views are from the side of the femur, and because you can 'see through' the bone the eye takes you past the rounded condyle and into the notch and then through the otehr rounded condyle, with the condyles appearing paler as there is less bone material there. The line drawn in red indicates the wall of the notch. The shape of the roof of the notch is relevant to the surgeon doing an anterior cruciate ligament reconstruction.
Extending the line to beyond the X-ray shadow of the patella (kneecap) also gives an indication of whether or not the patella is abnormally high or low, as it would normally intersect the lower pole of the patella.
The Correlation of Tunnel Position, Orientation and Tunnel Enlargement in Outside-in Single-Bundle Anterior Cruciate Ligament Reconstruction. Ko YW, MD, Rhee SJ, Kim IW and Yoo J-D. Knee Surg Relat Res. 2015 Dec; 27(4): 247–254.