The investigation does not need to be aimed at Trump for him to obstruct it.  I assume that Mike Flynn is one of the key targets of the investigation.  If Trump attempted to meddle in the Flynn investigation, which it appears that he did by firing Comey, then he was obstructing justice.

Additionally, while he was within in authority in firing Comey, he may not have been if he tried to pressure Comey to drop the investigation.  By asking the NSA and CIA to pressure Comey, he may have committed obstruction.  No doubt there's more to the story.