I was really shocked to see Blackrock and "low cost" in the same sentence. Then, I followed one of the links and found out how the plan is "low cost".
https://www.tsp.gov/investmentfunds/fundsoverview/expenseRatio.shtml
The published TSP expense ratios are calculated using net expenses after they are offset by forfeitures. Then, they compare it to the average expense ratios of other mutual funds. Anyone can look "low cost" if they exclude most of the fees and expenses they are paying from the expense calculation.