Spark includes its own version of Maven, so just go to the directory for the source code.
Build spark from the latest version using a clone of the github repo using the following line:
./build/mvn -Pyarn -Phadoop-2.7 -Dhadoop.version=2.7.3 -Dscala-2.11 -Pnetlib-lgpl -DskipTests clean package.
Build OpenBLAS from a clone of the github repo by running make in the directory where the repo is saved. Then use sudo make PREFIX=/opt/OpenBLAS install to install it in a directory. Add the directory /opt/OpenBLAS/lib to /etc/ld.so.conf.d/openblas.conf and run ldconfig to load the new libraries.
However, in the end I wound up using the line sudo apt-get install libatlas3-base libopenblas-base to install the files from the repository because I couldn't get Spark to recognize my version of OpenBLAS built from source. I may revisit this in the future with Mint/Ubuntu alternatives sourcing (ie, sudo update-alternatives --config libblas.so)
This page is very helpful in resolving problems getting Spark to use BLAS: http://www.spark.tc/blas-libraries-in-mllib/