I agree with Gregory.
Due Diligence is key to the success in buying unseen properties. Your Power Team is always the most important part of your business in real estate but buying unseen properties it makes it even bigger!
Some tricks I do is working with a realtor that will do some extra leg work for you.
Ex: take lots of pictures of each part of the house, the neighbor houses, the main streets. Even videos, if possible.
Also you will need the realtor to open the door for contractors, appraisers, inspectors, property managers, etc.
This usually means you need to throw some extra cash sometimes. Depending on the relationship with the realtor. My advice tho, is do not call any realtors unless you are ready to start putting in offers. Relationship building is key to success, and you calling up to discuss areas without knowing anything about the area. That spells out, not serious, to the agents. Don't waste their time, they are very busy people!
Also use the internet to your advantage, use google maps, bing maps, you can walk through neighborhoods as if you were there. If you see a lot of run down properties, chances are they are still like that.
To be successful, you have too be very organize for buying unseen properties. I would not advice first time real estate investors to jump right into something like this. But once you get good at it, the world is your back yard have fun with it.