I think nginx does unofficially support multiple http blocks but generally you would only have one.
The idea is that you have a single http block/context and then one or more server blocks within the http block. Within server blocks you can have one or more location blocks.
Configuration settings are inherited and can be overriden using the above nested approach.
Regarding sendfile, it can be configured within any block:
Syntax: sendfile on | off;
Default: sendfile off;
Context: http, server, location, if in location
Therefore you can simply add "sendfile off;" to your server block and this will override the sendfile value set in the http block.