One of the latest tools for fighting MT comment and trackback spam is Brad Choate's SpamLookup plugin. I have it installed here along with MT-Blacklist, and between the two of them, well over 99% of comment and trackback spams are blocked automatically.
Joe Katzman at Winds of Change submitted a SpamLookup ticket, asking for the ability to approve trackbacks blocked by SpamLookup. I e-mailed this reply to Joe's ticket:
Blocked trackbacks are literally blocked. They are not saved into the MT database at all, so there would be no way to later approve them.
In order to approve trackbacks, MT must have the ability to moderate them. MT currently does not provide the ability to moderate trackbacks, nor does SpamLookup. If you want to be able to moderate and approve trackbacks, you would need to install Chad Everett's MT-Moderate plugin, which can be downloaded from his web site.
SpamLookup is aware of MT-Moderate, so once you have MT-Moderate installed, it should properly moderate trackbacks if you configure SpamLookup to do so, and give you the ability to later approve moderated trackbacks (on the Trackbacks page in both SpamLookup and MT).
If you already have MT-Moderate installed, but still cannot moderate or approve trackbacks, that would most likely be an installation or configuration issue.
Moderation of trackbacks is a feature that has been long missed in MT3. MT-Moderate fills that hole nicely, and it's even better when paired with SpamLookup.