Armadillo doesn't prevent OpenBlas from using more cores. It's possible that the current implementation of OpenBlas simply chooses 2 cores for certain operations.
You can see Armadillo's source code directly in the downloadable package (it's open source), in the folder "include". Specifically, have a look at the file "include/armadillo_bits/fn_solve.hpp" (which contains the user accessible solve() function), and the file "include/armadillo_bits/auxlib_meat.hpp" (which contains the wrapper and housekeeping code for calling the torturous Blas and Lapack functions).
If you already have Armadillo installed on your machine, have a look at "/usr/include/armadillo_bits" or "/usr/local/include/armadillo_bits".