It depends on the welding job for one.. and just how bad the crack is for another. Most of the cracks that happen on the frame there are not bad... but can get worse. If the weld looks good, and it looks like the crack is not spread past the weld, or is not a major crack, then you can get the weld in brackets, and the bolt on brace, and I would imagine it would fine.
Are you happy with the rest of the Blazer? If so, then I would probably say go for it. If you're not sure about the quality of the weld, or the severity of the crack.. run it by a mechanic shop and let them look at it, and give you their opinion. If you can't get it there, maybe you could find someone to come look at it, where it's at.
Either way the crack can most likely be fixed... but of course a small crack is better to fix than one that has cracked time and time again because it was never fixed correctly to begin with.
Good luck.