Executing stress testing is a good start to find bugs. It might not be an ideal black-box testing method though as it is mostly for developers to test their proprietary software. Injecting delays at various points into the target could help finding bugs but we need to know the right locations to inject the delays. Cuzz is a Microsoft tool for finding concurrency bugs by injecting random delays - it looks promising.
Using DBI (Dynamic Binary Instrumentation) it's possible to tell if an EIP is executed, and if so by what thread(s). Therefore it's possible to tell what code is executed by what thread(s).
Using DBI it is also possible to tell where (value of EIP) the thread context switch happens.
By having the above information we can make educated guesses where to inject the delays.
If a bug is found it might not be reachable from outside. That's always a possibility. However it's good to see if you can provide input that makes the application to run longer near the location of the intended delay. There might be a
ReadFilethat can take longer to complete if the file is large enough. Or there might be a loop where the iteration count can be controlled by user...