One option is to use a category for just "this blog" stuff.
Use a stub file for your blog #1
Pick a few categories to use for "hidden" posts.
Figure out the IDs for those cats. (Click on the link on a post for that category, and then look at the URL, or look in the DB in the evo_categories table.)
Put this in the stub file for blog #1:
$cat = '-1 -2 -3';
where 1 2 and 3 are the cats that you want to hide.
When you post on those other blogs, make them a part of that category.

I got a blank page when I tried it that way, when I tried adding more than one category I received a blank page and the title bar has a message saying something about a category 0 not existing, but thanks for the help, I figured out a solution based on that, I just reversed the idea and listed the categories I wanted included instead of those I didn't want included (since I only use them for myself and don't have them displayed on the actual entries anyway, it's not that big of deal to edit the file if I add/remove a category later :))

I have a category that I want to hide, so I thought I would try this method.

When I did it for a single category, it did indeed, hide the actual blog entries, for that category, but I noticed:

1) The category name still showed up on the sidebar (with an * by it)

2) Any category selection (including ALL), had a title that read "Category Selection: [Category] but ['hidden' category]" (e.g. - "Category Selection: All but Hidden)

Neither of these behaviors is 'hiding' enough.

Also ... when I put in "$cat = '-2 -3' (more than one), it didn't work. No posts are shown & I got the message (as jt1980 describes) "Requested Category 0 does not exist!"

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Choosing the inverse: (selecting all BUT the one I want to display) yields:

1) A "Category Seleciton: [category selected], [category selected], [category selected], .... [all categories selected]" , which is undesireable.

2) Category listing in the sidebar that INCLUDES the one category that I want to hide, with all but that category containing an asterick (*) by the name. (also undesireable).

This method didn't "hide" the category as I would like, but I thought I'd bump the post with a description of what the solution yielded.

(please don't ask why)

Why jt1980, why?
sorry, couldn't help myself.

I am working on the same problem. I have a blog with all my categories in it and a seperate links blog with 1 category called links. I am currently using categories across both blogs, but on the main page I don't want posts with links category to be shown. I tried the suggestion $cat="-75", but it only removes posts with only that category. Is there anyway to make it so that a multi-category post will hide when this is done? Point me in the files direction and I'll try me best to hack it. Suggestions, ideas?