Blake Stone: Aliens of Gold

Why does this game have a ESRB rating when it was released a full year before the ESRB exitsted ? From looking at the cover art I can't even find a RSAC rating.

It's probably from a re-release.

Yup, it must be from a re-release. If I recall correctly, and I'm almost positive I do, the game had a "voluntary" rating, much like the PC-13 rating for Wolfenstein 3D.

Here, take a look at the 3D Realms website. Scroll down and they show the game as having an M rating, although as you correctly pointed out the ESRB didn't exist at the time of the game's release.

LINK.

Ah.

Then shouldn't there at least be one box or something showing the rating ?

It's possible that it wasn't released in a box anymore after the ESRB rated it. But the rating is on the publisher's site for the game.