If at all possible, use 301 redirects for the old blogs so they redirect over to the new blogs.
Easiest way would be to use the same style of permalink structure and using a RewriteRule in your .htaccess file.
If this sounds like jargon, look up each term on Google!
I think the hardest thing will be actually migrating the blogs.
Depending on the blogging system you’re migrating from, this could be a challenge!
If the old blogs use WordPress, you could get away with using a database export or by using Ron Rennick’s excellent Advanced Export plugin.