Are there any specific design considerations that are made over nonreusable, traditional rockets to tolerate the blast from the stage 2 engine? As shown in this moment:
The top of the first stage (booster falling to Earth) has a pusher arm which pushes the second stage away from the first stage. I assume they have calculated this to provide enough force to create a big enough gap that this doesn't affect the flight of the first stage. You can view pictures of the first stage once landed, there never seems to be any extra wear and tear near the upper part of the first stage. This indicates that its not largely affected. The distance between the two stages is probably considerably larger than it looks, would work it out but the force the pusher applies isn't known outside SpaceX, so no conclusive answer, but I think its safe to say it won't affect re-usability or they wouldn't do it. Hope this helps.