The 1860 Louisiana Census list the slaves owned by William Silliman by Silliman, sex, and approximate date of birth. However, I cannot determine their given names on the 1870 Richland Parish Census.
Here is a list of the names of African Americans from the 1870 Federal Census of Richland Parish though I am not sure of their status:
Also, here are some other resources to check out:
Even though this isn't the correct county, here is a lot of helpful information and links to resources for an adjacent county that could set you in the right direction for finding your ancestor.
This is a searchable database for people in Richland Parish around that time, if you have an idea of names or you can search the William Silliman and see if you can find other information that is helpful.