As for the Blackstar, you could retube that thing so cheap and even improve the tone substantially. It would only be like $70 after shipping.
As for the speaker, if you're set on just replacing the speaker, Celestion Vintage 30's really are the quintessential rock guitar speaker. They just have the best balance for really anything you want to play, in the opinions of many, many players. But personally, if I were you and had a little extra cash, I would just buy a whole separate cabinet since your amp has separate speaker outputs. A separate cab is something you could use with future amp purchases and you could dramatically improve the fullness in your tone. I'm all about using 4x12 cabs personally, even if you aren't playing at super high volumes. They still give you a much tighter and bigger tone and can handle the extra wattage should you ever purchase a bigger amp head. Even a good 2x12 can yield some great results and would obviously be more portable than the 4x12. There is a lot to consider depending on what goals you are aiming for.
Zilla cabs out of the UK are highly recommended and aren't overly priced. They make high-end clones of other popular cabs and also have a few original designs as well. The Mesa Boogie 4x12 Standard Oversized is probably my favorite cab of all time, and those can usually be found pretty cheap on the used market.
If you want any more help making your gear purchasing decisions, I'm always willing to live vicariously through you and your credit cards!