Step 1: Dimensions.
\( [\sigma_s] = MT^{-3}K^{-4} \), \( [k_B] = ML^2T^{-2}K^{-1} \), \( [b] = LK \).
Step 2: Combine.
\[ [\sigma_s k_B^{-1} b] = \frac{MT^{-3}K^{-4} \cdot LK}{ML^2T^{-2}K^{-1}} = L^{-1}T^{-1}K^{-3} \]
Step 3: Final.
\[ \boxed{[L^{-1}T^{-1}K^{-3}]} \]