You don't say, but I'm guessing (best case) you bought the new battery, popped it in, and the BlackBerry still wouldn't turn on. But you plugged the BlackBerry in to a charger with the new battery and the BlackBerry came on after a while. If that is the scenario, then the new battery was discharged and the BlackBerry came on once the battery became charged enough.
If the BlackBerry with the new battery was not on a charger as discussed above, then it sounds like there is an issue with the BlackBerry. Have you dropped it hard, or gotten it wet?
well the guy at the store put the new battery in, said it should turn on and it didn't. so I didn't buy the battery (didn't want to spend $50 if it still wasn't going to work). I got it back home and put the old battery back in, and that's when it came back on. I haven't used the phone in probably a year, but I don't remember dropping it really hard or getting it wet. it seems to be working now, I'm letting it charge and am hoping it was just from not being used for a while.
I'm not sure what the problem is. If you charged your old battery between the time the the BlackBerry wouldn't turn on and then the time that it did, that probably explains how it turned on. And in that case you may find the old batttery runs down very fast, or the BlackBerry reboots when there is a high power demand (like phone callls). Or, it may be an intermittant problem with the device as you suspect.
There have been cases in the past where people have found the battery is loose in the compartment and intermittantly loses contact with the terminals on the device. That typically results in the BlackBerry spontaneously Rebooting, not shutting off and staying off. Also, you don't see that reported nowadays, and was probably associated with a particular series in the past. But check your BlackBerry anyway, make sure the battery fits secure in the compartment.
All that said, if you keep having issues, try a battery that you know is good and see what happens.
I do recommend a new battery although you are correct it may be a device issue if the same issue can be duplicated with a new battery.
I did pull this information from a 3rd party site
The battery icon on your BlackBerry usually tells you when it's charging and how much charge your phone has, but you might occasionally see the icon with a red "X" over the battery. This can indicate your battery is dying or that your phone is having trouble reading it. Clean the contacts on both your battery and phone to resolve the problem. However, this won't solve the problem if your battery is actually failing.
Let us know if that new battery works or your battery continues to work once it was charged./discharged then charged again.