There are no it8 targets for negative film. The idea is that if you use any transmissive it8 to make yourself a profile, you can use it on your negative scans. The negative will have more accurate colors this way. You know, red is red even on a negative until you make it into a positive.
Basically same idea if you would make a profile for the reflected scanning, anything you scan on it will then have correct colors. Just that positive it8 targets took this a step further when every film had separate IT8 targets for perfect output.
Overall it's always good to have a profile for the hardware, otherwise it's hard to convert the file into any other profile like for editing, paper profile for printing or for web.