That's left over from GDR2. GDR3 does not, I repeat, does not use "other storage", but it also doesn't clear whatever you accumulated in GDR2, so once you update to GDR3 you have to hard reset to reclaim the lost space. As such, once you've hard reset after updating to GDR3, nokia's storage check does not give accurate or reliable results.
At least, I have seen zero confirmed cases of "other" truly growing in GDR3, that was not accounted for and deletable in GDR3's storage tool. All threads about "other storage in GDR3"- it's always really just left overs from GDR2, and users misinterpret this as "it's not fixed". Or users are still using nokia's storage check, and they mistakenly think the problem is still there.
As far as lumia black goes, I think it's likely the update isn't cleaning up installation files and OP might need a hard reset. But I highly doubt the claim that "other storage is growing" because, once again, this information is gathered using an outdated and unreliable tool. Unless nokia reintroduced the other storage problem.