Explained in the books. It wasn't strictly speaking a 'blind' jump, as Cortana used a loophole in the Cole Protocol to make a jump that was still away from the Inner Colonies (so following the rules), but based her 'randomized' jump vector on co-ordinates she translated from Forerunner relics found on Sigma Octanus IV by the Spartans (the site of their, and Jacob Keyes', last battle before the fall of Reach). Those co-ordinates, which only Cortana (and later Doctor Halsey) knew about, led the Autumn to Halo. Cortana didn't even know there would be anything there, it was basically curiosity on her part and the idea that the Covenant were after something there (given their interest in the Sigma Octanus IV relic), which coincidentally happened to be true.Platinum117 said:How did the Pillar of Autumn find Halo if they made a 'blind' jump? Though i think it may have been explained somewhere...
The humans, of course, put it all down to sheer dumb luck (well, your mileage may vary on the 'luck' part), which is also a fairly good and sensible possibility for gamers to use if they don't know the backstory, if a little unlikely.