While sorting my LEGO collection I found bricks with two different fonts and many without a font at all. I also found Tyco and Mega Bloks bricks. So did LEGO ever make blank bricks?
Yes and no. If you found normal basic bricks without the LEGO logo, then they are definitely not Lego.
However, some parts are too small to have the logo. Things like utensils from the Friends theme, balls, technic pins, etc. don't have room for the logo, but this doesn't mean they aren't made by Lego.
The very first Lego bricks (2x2 and 2x4 bricks in 1949) also didn't include any logo or markings on the bricks. They also had completely hollow undersides, and slots in the side walls. But I suspect this is not what you have.
As for the bricks with two different fonts of LEGO, these may be made by Samsonite (under contract with Lego) in the 60s–80s. They were notoriously careless with keeping the logo straight on the stud, or even having the same type of logo on multiple studs on the same brick :)