Is XORing a random stream of bytes a secure way to obscure a file? Long ago I created a very simple program that used the C library's rand method to generate bytes to XOR with the bytes of a file. This was a reversible (just run it through again) method to make a file unreadable.

In my simple implementation, I did not change the seed, so it always created the same "random" stream, but if I had given a seed as a command line argument, would this be a reasonable way to obscure a file?

This question is not about the merits of any particular pseudo-random number generator, just assume a good one. The point is: does it provide a sound obscuration using a very simple algorithm?
XORing a plaintext with a random stream of the same size is a secure way to encrypt it. A stream cipher works in this way. 

The issue in your design is the random source. If the random stream does not have the security properties one expects, the systems is not secure. 
In your implementation, the seed has the role of the key (and potentially the initialization value) of a regular stream cipher. 

If someone can encrypt something with your method (or recover a couple plaintext/ciphertext) he will be able to decrypt every past/present/future encryption generated with the same key. Only for messages shorter than known one (but the remaining part could be guessed if the file is, e.g.:, an english text).