I'm not fimilar with Trulia's site. Everybody's defention of a bad neighborhood is different. Drive around these areas at different times to see what's going on. If your managing yourself it, is this a place you feel safe, not to mention bringing family and friends. Some areas it's spotty and a block away things are better. Do your research of the neighbourhood for crime stats, rentals versus owner-occupants, sometimes cities provide this information or other data sites.
My first rental was in a questionable area, and it was quite a learning experience. It was a combo of area, bad PM, and me being a rookie. I kept it when most others would probably of run. First situation wasn't good for the tenant and a friend of his. PM threw up there hands after putting a questionable tenant in, city started coming down on us. After that we had a few evictions with drama, spent more money fixing damaged things that shouldn't of had to, belongings left behind, break ins, loss of rent etc etc.
It definitely stopped me from buying other properties, because this one was a suck of time and money, that I believe was a mistake. On the other hand I still have it today, stable tenants now, and area is seeing some redevelopment. Thanks to great handyman and new PM that have helped me along the way through these troubled spots.
Not only screen your tenants but also your contractors, don't get sucked in by their drama. Have money set aside for when things go awry. Good tenants do go bad and it doesn't always matter to the neighborhood, be a active landlord.
Network on BP!