OK, now I understand what is going wrong:
When you start, the area of the blood event is the same size as your video. When you zoom-in on the blood, the whole screen will be filled with the white event and the blood. So still no problem. But what you do is to zoom-out. And then the area of the blood-event is smaller than your video below. So you introduce a black area in the blood event. And that indeed masks the event below because of the multiply mode.
You better do what John already suggested:
Add your video on the lower track. Add the blood-event twice, on two tracks above the video. Add the secondary color correction tool to the upper blood event and set the saturation to zero. Also add the mask generator tool to the upper track, choose the luminance option and select INVERT. Set the uppertrack mode to multiply (MASK) and make the middle track (also with the blood event) a child of the upper track. That gave the best results to me. (see picture)
Unfortunately, the footage you downloaded does not contain an alpha channel. Introducing an alpha channel with the secondory color correction tool is possible, but it looked awfull. So the problem with zooming-out remains.
One possible solution to this problem is: Open a new project, set the event with the blood on track 1, and a white generated media event on track two below the blood. Perform the zooming-out on the blood event and render this project to a new file. Use this file in your main project in the way described above.

Theo