Could I configure IntelliJ diff viewer/merge tool as external tools in tortoiseSVN?

Re-read Comparing files using intellij idea as a diff command line tool until there will come enlightenment in getting correct command-line

<IntelliJ IDEA launcher> diff <path to file1> <path to file2>

where file1 is your local copy, file2 is the repository version.

for external diff for TortoiseSVN, there you have to use %base (repository-side file) and %mine (modified file in WC) variables as placeholders for compared files.

You addded nothing (nor command, nor parameters) into your command line, thus - got nothing

Answer

/path/to/idea diff %mine %base

If you want to configure the merge tool, use this configuration:

/path/to/idea merge %mine %theirs %base %merged