Actually I’d say both combinations of primary/secondary work with blade+pistol, but there are a few things you have to keep in mind.
Pistol is one of the weapons that work just fine both as primary and secondary.
For blade, if you want to get good DPS out of it you need to use Spirit blade mechanic which eats up one slot for the ability. It thus makes most sense to have Blade as your primary weapon for decent dps.
However, Blade can be used fine as secondary as well if you mainly use it for self-heals - and blade provides some very nice self-heals. Not so good for dungeons, but can be very nice for solo play.
To make a long story short, for solo play all weapon combinations are viable and it is hard to really mess up a build. So don’t worry too much about your build - if you have trouble with anything, that is when you can experiment with different builds to see what works best for you.
For group dungeons what weapons to use depends on your role, and some weapons are simply better than others in some ways - but a half-decent build is not all that hard to put together here either if you just spend a few minutes reading what the abilities do.
Blade is usually not recommended for dungeon runs, because while its dps is no better than that of other weapons it provides less group utility. It still is viable as long as you aren’t trying to push the envelope, it is just that there are better options.