While that request seems easy enough: it won't work.
The blame viewer doesn't know which is the "next" revision. While the "previous" revision is always one less than the current, the next one isn't just one more than the current.
The current revision in the blame viewer for each line is always the revision where a change occurred. So one revision before, the line looked differently.
But the next revision rarely has a change to that same line or even the same file. It might take another 100 revisions until that same file gets changed again.
The same explanation as above: if there is no 'previous' revision anymore for that file, the blame viewer can not know that beforehand.
But yes, the error message could be different. But that would have to be done in the svn library, not TSVN.
Stefan