Blocking sites from showing up in the Google search results no longer works.
First reports came a week or so ago via Google Web Search Help where Googler, Kousha, said that "this was a known issue a while back." Well, it still seems to be an issue.
I tried to replicate it and I was able to two different ways.
(1) First, when you click on a search result and then click back, Google is suppose to show you an option to block the result from showing up in the future. That did not come up for me this morning.
(2) Second, when the first option did not work, I went directly to blocked sites search setting, added www.proflowers.com to the list and waited a minute or two.

I then searched for [flowers] and the first result was www.proflowers.com:

Earlier this year, Google had a bug where you were unable to unblock blocked sites - that does work but you can no longer block sites in Google.
Forum discussion at Google Web Search Help.

Comments:
Andrius
12/13/2012 03:24 pm
I was looking for this feature few days ago too - domain crowding is so alarming it's not even funny.
Nell Terry
12/14/2012 07:12 am
Maybe it's not blocked because it's an ad? Perhaps Google only allows you to block sites from appearing in the organic results. Have you tried it that way?
JIMMY
12/14/2012 04:22 pm
I dont gues we're talking about the Remove URL feautre in webmaster tools ? Because I've used that recently and I think it works OK but Google takes its time removing old unwanted Urls that still linger in caches. So if you dont want to see a web site in the SERPS for some reason you can request that it not be shown on your computer in future SERPS ? ok....didnt know that. assuming you're logged in.
CandleForex
12/15/2012 08:22 am
Sorry...what is domain crowding?
Andrius
12/15/2012 12:21 pm
When you see 3 or more results from the same domain as different search results,
CandleForex
12/15/2012 11:29 pm
Oh thanks! I did not know that
Guest
12/16/2012 04:14 pm
I'm searching to try to see if there are any discussions/write-ups of comparisons between two NBA players, and literally the entire first page of results is from the same site, which is just some fantasy basketball website that has some awful algorithm to determine if you should make certain trades, so all the pages are just things like "Trade Kobe for Mark Gasol and Josh Smith" That's not what I want... blocking search results, not working though.
BBadger
12/29/2012 08:13 pm
It's been broken for months after working again for a short while before that. That eHow pollutes so many of my search results. ARGH.
travis
01/09/2013 02:57 am
agreed, this needs to be fixed so once again experts-exchange can get the effffffff back out of my results! I'm NEVER going to pay for something the rest of the internet offers for free.
travis
01/09/2013 03:01 am
this is unavoidable as more informational sites continue to grow (like mailing lists, try researching source code issues for syslinux you're going to find lots of zytor mailing list results because all of those email exchanges are stored on that domain but are all different potential results) or how about stack-exchange, i get a bit of crowding from there but it's quite pleasant as their format is fantastic and the accepted answers are often most useful
charlieprimero
02/08/2013 11:44 am
Blocking sites from Google search results does work for some sites. "Manage Blocked Sites" will not work on domains which pay Google such as fixya, ask.com, http://www.google.com/reviews/t
Yoda
02/17/2013 10:17 pm
Seems like fixing this isn't a priority for google. Would be interesting if a competitor implemented such a feature.
=/
03/01/2013 10:24 pm
You're an idiot, go away.
JIMMY
03/02/2013 01:36 am
No you're an idiot Sent from Jimmy's iPhone
btraven
03/02/2013 06:25 am
I just recently noticed this. What I've been doing is put the site name in the search box with a minus sign in front of it. Seems to work.
kylepfeeley
03/05/2013 03:16 am
If you use Chrome you can use this Chrome Extension https://chrome.google.com/webstore/detail/personal-blocklist-by-goo/nolijncfnkgaikbjbdaogikpmpbdcdef?utm_source=gmail