Reportedly on
chinadaily.com.cn/

etc.

baidu.com/

google.com.hk/logos/white_logo ...

Also see:

baidu.com/img/baidu_logo_jr_10 ...

And Bing

bing.com/fd/hpk2/BlackCandle_Z ...

And Yahoo

cn.yimg.com/hp/100420/yahoolog ...

Sogou

sogou.com/images/i_black.gif

You are so proud to share your discoveries but no one is glad enough to explain WHY all is black & white!

=> It's because it's a national mourning day in china due to the deads in the floods

google.com.hk/intl/zh-CN/logos ...

There was no good references yet when the first forum posts were posted. Personally I gathered information via local news (using AFP as source).

This happened in May 2008 too:

'Chinese websites turn sombre as nation mourns earthquake victims'
forbes.com/feeds/afx/2008/05/1 ...

My source in this month:

yle.fi/uutiset/ulkomaat/2010/0 ...

Examples are from search engines etc, there is China Daily's page:

yle.fi/ecepic/archive/00339/ki ...