I’d say Blink probably isn’t the product for this. To detect a small bump like that, you probably need an accelerometer. Your iPhone has one built-in. I’m not sure if they make internet-connected “smart” accelerometers or not. If so, you could theoretically trigger some kind of camera to start recording when the accelerometer detects a bump, but it all depends on the features of the product(s) you’re using. Blink, for example,. doesn’t have a way for you to start recording programmatically. (Unfortunately.)
If you’re a programmer who knows Python, you can probably figure out a way to do it by triggering a live recording, but that’s probably too advanced for most people. I’m a web developer, and I’m looking into those capabilities for my own purposes, but it’s definitely not consumer-friendly. And you’d still need a separate device (internet-connected accelerometer or something) to be your trigger.
If you have an old iPhone you can keep plugged in and connected to the internet all the time, it’s theoretically possible, but you’d also need to be able to write an iPhone app to access the accelerometer.
Basically, if you’re a talented developer, it’s possible. If you’re not – probably not.