/usr/share/applications/screensavers/blocktube.desktop is in xscreensaver-gl-extra 5.30-1+deb8u2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | [Desktop Entry]
Name=BlockTube
Exec=/usr/lib/xscreensaver/blocktube -root
TryExec=/usr/lib/xscreensaver/blocktube
Comment=Draws a swirling, falling tunnel of reflective slabs. They fade from hue to hue. Written by Lars R. Damerow.
StartupNotify=false
Terminal=false
Type=Application
Categories=Screensaver;
OnlyShowIn=GNOME;
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