There's also the tag place=city_block [1]
which may be useful depending on its coverage but which has the advantage of being explicitly assigned by a human.
I've always thought of city blocks as an anti-pattern in urban layout e.g. exploring New York on foot for the first time I tried 'using my nose' to find interesting places but had the problem that I got zero incremental information until the next intersection at which point the large block size meant it was too far to turn back.
I prefer to live in cities for which the concept of a 'block' has less relevance, and I'd caution against trying to 'blockify' cities worldwide.
I've always thought of city blocks as an anti-pattern in urban layout e.g. exploring New York on foot for the first time I tried 'using my nose' to find interesting places but had the problem that I got zero incremental information until the next intersection at which point the large block size meant it was too far to turn back.
I prefer to live in cities for which the concept of a 'block' has less relevance, and I'd caution against trying to 'blockify' cities worldwide.
[1] http://wiki.openstreetmap.org/wiki/Key:place