How can I 'blow up' small pixel art?

Yes, simply use "Nearest neighbour" as the resample algorithm in the "image size" dialog (image -> image size)

Edit: @CAI gives a nice tip - "It's also worth mentioning, if you don't want any distortion at all, multiply the scale by whole multipliers (so 2x, 3x, 4x or 200%, 300%, 400% etc.)"