separate table vs. array-like list field

I'm very, very new to mysql and I've been toying with it the last
couple weeks. I've hit upon a theoretical question that I would like to
run by more experienced users.

One of the things I've been doing to teach myself is creating databases
and tables that might be similar to databases powering big, popular
sites like Flickr or Digg. For example, I created a user table (id
[primary key], name, nickname, email, location, birthday, profile,
etc.) and then played around with querying it in various ways.

One of the things I did to learn how to use "join" was to create a
separate table for blocked users. Sites like Flickr and Digg often give
users the ability to block other users. The way I implemented this was
to create a table with the blocker's id and the "blockee's" id. Using
that, I can query the blocked users table and cross-reference it to the
user list and come up with the list of blocked users for any specified
user. Pretty cool.

So I wonder. On a big site with a million+ users, would having to query
a separate table for a list of blocked users like that bog down the
site or would it be preferable (in terms of speed and efficiency) to
add a field to the user table that lists the id numbers of that user's
blocked users, sort of a pseudo-array that I collapse and expand when
needed (e.g., a blocked user array-like list might look like
"150|11516|61878|9091|41064" etc.

Any thoughts? Or am I still so new at this that this scenario doesn't
make sense?

--Rick