Bought iPhone 3GS with iOS 5.1, can downgrade to 4.1, any hope of going to 4.2 or 4.3?
I'm surprised that you found an SHSH blob for a firmware that low. Consider that a treasure. Unfortunately, though, you can only install the version of iOS that corresponds to the SHSH blob. The blob contains a signature that is unique to the device and unique to the iOS version. It is like the key to your device for that version. It can't be faked, but it can be saved. So in short, you're stuck with iOS 4.1 or 5.1.1, and there isn't anything that can be done now to change that (besides, the change from iOS 4.1 to 4.3 only added AirPlay/Print, Safari's Nitro JavaScript engine, iTunes Sharing, and Ping; which shouldn't be big enough features to worry about unless your app integrates AirPlay).