This. I wonder how no one mentioned this soonerGladly I have New 3DS XL. I can hold Select button and load DS games in original resolution without blur. Same goes with flash cards.
You can hold Start or Select when launching anything that is not a 3DS game to launch in them in their native resolution. All the official Virtual Consoles and injects (except SNES, where you can set scaling in the settings), DS games and DSi Ware support this feature.
DS/DSi isn't the greatest pick for emulation, because the resolution is too low for non-handhelds, so you either get a squished or cut off image. Gameboy (Color) works well enough, the GBA emulator is a POC, the hypervisor, gbarunner2, works better but also isn't perfect (visual bugs, sound bugs), Neo Geo Pocket (Colour) seems to have improved over the years.
But for home consoles... You're outta luck. NES is too high res and the emulator doesn't emulate everything yet, so no extra sound channels in the Japanese Castlevania 3, the DS is too weak for SNES emulation and the resolution is also too high for it, same for Mega Drive/Genesis. I couldn't get Turbo Grafx 16 to run and Neo Geo works surprisingly well, but you'll run into resolution issues again. And that's pretty much how high you can go.