Up to now I worked with command line blastn results without seeing any indication for the results being on the + or - strands of a DNA. Is there a parameter to pass with the blastn command in order to get results for both strands?
If you don't specify it specifically, the default of blast is to search for both strands.
It will also print in the output if it's plus or minus (in the default output at least it should mention "strand: plus/plus" or such). You can also always look at the alignment coordinates and if those are small -> big it's plus, if it's big -> small it's minus strand