The Blackbox i10s do a pretty decent job of shutting out noise passively, as the silicone tips give a good seal. Start playing some music, however, and the active noise cancellation kicks in and delivers a much more comprehensive level of isolation. As ever it's much more effective with low-frequency 'humming' noises - such as the office air conditioning unit - than with the sporadic noise of, for example, typing on a keyboard. That said; the attenuation is good enough that you'll definitely want to look both ways three times before crossing a busy road.
The effectiveness of the i10 earphones' noise-cancelling would be all for naught, were they not to also sound pretty good. Fortunately, they don't do too bad a job. There's a definitely over-emphasis on the bass end of the spectrum, but it's not overwhelming - and many listeners will prefer that bias, anyway. Mid-range clarity is pretty good, with decent detailing and no noticeable distortion, but the high end is the weak spot with cymbal crashes in particular falling prey to sibilance which can mar the whole experience on some tracks.
So we're not talking Grado levels of clarity and detail here, but the i10s are in the same ballpark as other noise-cancelling sets in the same price range, such as the Creative EP-3NC earphones. Obviously the utility of the Blackbox i10 noise-cancelling earphones is limited by their use of a dock connector. We reckon that for iPod owners the practicality of not having to find batteries to power the earphones more than makes up for the inconvenience of not being able to use them with another device.
While there are other, better-sounding sets available for the money, the noise-cancelling properties of the Blackbox i10 earphones' and their neat trick of connecting via your iPod's dock port gives them something of an edge over the alternatives.