In older versions of current browsers (eg, Mozilla), it was possible to block specific cookies from being set (in the window where cookies could be individually removed).
In Firefox and Safari, as far as I can tell, this is not possible; cookies can only be removed individually or en masse. There seems to be no way to prevent cookie intruders from inserting themselves.
Blocking cookies from unwanted sites such as doubleclick and other advertisers would be profoundly desirable. My previous list of blocked cookies extended into triple digits.

Can cookie blocking in fact be done? And if so, how?

Here are some things that you should look into:

Ghostery
Glimmer Blocker

I assume that you have played with the settings in Safari's Preferences, Privacy pane. Click the Details button for more information.

You assume correctly.

I'm not looking for ad blockers, just cookie blockers. And I can't just block all cookies (even if it were an option) because I need them to be allowed on numerous sites.

So there's nothing internal to either Firefox or Safari which would do I want to do and did in my previous iMac's archaic browser? Ain't "progress" grand?

Lion compatibility of them.
Isn't that statement a bit premature? Anyway, have you seen this Firefox Help page on blocking cookies?
For Safari there are the following freeware utilities: Safari Cookies, SafariCookieCutter, and, perhaps, Cache Out X. Unfortunately, I'm not sure about the Lion compatibility of them. Finally, you may want to look into how to enable Safari's "Do Not Track" support in Lion.

Merci. And no, I don't think it's premature.

I hadn't seen that Firefox help page, but now I have. That help page does not help with the latest version of Firefox, 7.0.1; it describes things which don't exist in this version.

Two downsides:
One can only block a site with considerable time and effort compared to the previous method of just blocking cookies in the list of cookies that had been set and remained in cache.
One still cannot block cookies that a website uses to deliver other material (eg, adserver, doubleclick [in all its various incarnations]); the browser doesn't seem to recognize them as third-party cookies. Again, the previous method allowed such.
C'est la vie.

From the website of Ghostery's developer:

Ghostery allows you to block scripts from companies that you don't trust, delete local shared objects, and even block images and iframes. Ghostery puts your web privacy back in your hand.

I don't know if that translates to cookies, but I use Ghostery and it appears to do what it promises.

> I don't know if that translates to cookies, but I use Ghostery and it appears to do what it promises.

A while back I e-mailed the Ghostery folks and asked them how, inasmuch as what Ghostery blocks isn't "there before and gone afterwards," I could determine that Ghostery was actually doing something.

I'm still waiting to hear back.

Anybody ?

Safari Cookies enables the easy deletion of unwanted cookies, but it does not allow you to block cookies.

I follow MacUpdate pretty closely, and if there's an app that allows you to block cookies I've not only never noted it, but I, too, would like to know what it is.

As it stands, I use Safari Cookies to set "Favorites" and delete "Non-favorites" as often as I think of it. (SC can also be set to delete "Non-favorites" when Safari is quit.)

Correct, but since there is a degree of automation in removing certain cookies upon quitting Safari, it effectively resembles blocking.

Retroactive blocking, so to speak?

This may sound elementary but "How do you do that?". I've looked everywhere (I think) through my Safari Preferences and can't seem to find a place to make those settings.

Safari Cookies is a separate app from Safari, not part of Safari's preferences.

Thanks......I'll take off the dunce's cap now.

I have noted – with considerable satisfaction/delight – and am pleased to report that by not checking/enabling "Accept third-party cookies" in the Privacy panel of Firefox's Preferences, cookies that used to have to be blocked manually (in Mozilla browser) no longer have to be (ie, they just don't show up).
Excellent. (And my previous concern has been thereby allayed.)