in case the download ever goes down I'll post an explanation:
the vpk contains a "materials" folder. In materials you'll find:
"healingtrail.vtf"
"softglow.vtf"
a folder called "smokelit2" in which there is "smokelit2.vtf"
All 3 VTFs are empty 1x1 images. VTFs are just images with some flags attached for source engine properties
the game attempts to reach this VTF. It does this by looking for a VMT file of the same name (a txt file that tells the engine how to handle a material). But there are no VMTs here, only the VTFs. So the source engine cannot load the VTF and replaces it with the default texture