I'm a little confused by your wording here - it's kind of all over the place. But I'm going to make some assumptions and guesses as to what you mean - so if I'm incorrect, then please let me know.
I'm *guessing* you already have a blog installed. Using your example, it's located at yourdominaname/blog1 ("blog1" being the actual wordpress directory, and contains all of the wordpress files.) You wanted to be sure you associated the database to the blog in an easy-to-remember way, so in the wp-config.php file, you changed the "table prefix" link from "wp_" to "wp_blog1"
Am I right so far?
Going with the above assumption, now you want to install a new version of wordpress, so you will have *2* wordpress installations on the same server, but in different directories. For purposes of example, you want it to be located at yourdomainname/blog2 (and you're still keeping yourdominaname/blog1). You want to use the same database for both blogs.
Now, if my layout here is correct, then your question of "is blog1 prefix reusable?" - the answer is a big, fat shiny NO. Not unless you want to overwrite blog1 with blog2.
If you want a second blog using the same database, then you need to change that tasble prefix in the wp-config.php file to "wp_blog2" and save it.
Then, when you install, your "blog1" will still remain, but it will write *new* tables with the "wp_blog2" prefix - leaving your "wp_blog1" prefixes untouched.
Hopefully that answers your question.