If you have a low budget and are just starting off, I would recommend ScrapeBox any day of the week.
ScrapeBox is more than a Blog Commenter. It has additional addons that can make your life a lot easier both regards to common and customized Blackhat and Whitehat methods.
SB can also scrape thousands of keywords in minutes, and I mean minutes. This also is beneficial to those who plan to create several micro niche sites. It doesn't necessarily "spam" blogs, it posts comments that you put in. The success rate depends on your usage of the software; such as the proxies you use, including your comments.
Xrumer, is far more powerful that ScrapeBox. Xrumer has the ability to manipulate forums to seem as though it's populated, it can build thousands and thousands of Backlinks on auto pilot, and when learnt, it can do much more than just post on Forums.
I can't comment on Black Links Pro. Although, if you have a small budget and limited amount of time, go with ScrapeBox. If you have a larger budget, time to test, tweak and learn, then go with Xrumer.
But hey, Black Links Pro can be the best thing since sliced bread. But, I don't know, and I haven't seen any reviews yet either.
Hope that helps!
All the best and good luck,
Lynn.