#1
Hey UG.

I have just completed learning blackbird by beatles. The first tab-song i have learned, and now i have a question; What should my next song be? I want to challenge myself, and take the next step from blackbird.
#2
Id advise that you just learn any song that you love. The best way to learn guitar is to have fun with it... as well as having those stressful sessions when you learn harder theory stuff or hours of playing something to try to get it down...

A few good songs to learn would be:

Guns n Roses - Don't cry (Acoustic or Electric)
Andy Mckee - Drifiting ( Acoustic)
Andy Mckee - For my father (acoustic)
Gary Moore - The Loner ( Electric)
Eric Clapton - Layla (Electric)
Eric Clapton - Tears in heaven (Acoustic/electric ---- This one is a good one to carry on from blackbird)

And anything else you enjoy listening to..

goodluck!