Is he still being Blackmailed?
If he has been blackmailed in the past, he is no longer under threat (unless who ever has negative information on him just want to destroy him).
Let us assume he has been blackmailed, and let us assume that what ever it is is released to the public, what would the results be...
1. He sucks it up, explains his side of the issue and remain on the bench, no longer afraid of exposure.
2. he sucks it up, explains his side of the issue and resigns from the court. President Trump appoints a new Justice.
3. Those with the information decide not to use it, he realizes that their knowledge is no longer a threat and he begins to rule according to the constitution. A win for us.
The only bad outcome is if he is such a moral coward he remains under the control of his (supposed) blackmailers.
Of course Justice Roberts could just be one of those guys you can not figure out and will vote how he “feels” on an issue.
Do you not know why we think he is being blackmailed?
I think #2 is the most likely.