> From the tutorials, it seems that io-winmgr is just a server.
> 
> I start io-winmgr with default config file, just get a black screen.

Yes, that is right, you can also specify a different colour if you so wish through the graphics.conf file.
 
> If my understanding is right, to make composition work, there must be a client
>  app just like photon to provide a window system.
Yes there must be a client app that is running. But not Photon. You need to create client app(s) to run that use the 
OpenKODE spec. Composition Manager implements part of that spec to allow for the creation, deletion, and managment of 
windows.

> Is there a window manager on qnx provide a environment like composite desktop,
>  on which we can do rotate and reflect?

These features are not implemented in CompManager at this point.