I don't mean getting CC chained to make you LOSE the blood rush. I mean "Oh, Cho+Leona stunlocked me so I couldn't get my spells off".
In the situation you described, you wouldn't lose blood rush. Missing 1 second of combat and then getting stunned for two seconds refreshes your combat, so you have two seconds. Even we interpret that as the other way around, that's 3 seconds.
Btw, if this ends up being a major problem for you, keep in mind that DOT effects will continually refresh your blood rush, even if you are reviving. Red buff, corrupting pot, sunfire, cinderhulk, Skirmishers, and ignite can do this well. If you don't care about your build, Liandrys, Redemption, and DFT can probably do it as well.
Aatrox ult has a radius of 550.
For comparison, that's the same as Amumu ult.
Yes, Bandage Toss is a better tool for getting in the middle of things than Dark Flight, but you don't NEED to get in the middle: you need to clip two members of the front line.