Are Blue Shift and Opposing Force "canon"?

There is no official Half-Life canon. Valve have not and do not intend to define what is and what isn't canon. Series' writer Marc Laidlaw stated that they do not "get involved in issues of canonicity", that "canon itself is non-canon", and that there is "no official stance".

Source: http://half-life.wikia.com/wiki/Half-Life_Wiki:Canon