You could overclock on the stock CPU cooler, but not very much, and that'd be really pushing it. I recommend you get an aftermarket cooler even if you want to do some mild overclocking, Intel's stock HSF just sucks. A great cooler for $34.99 off newegg would be the Cooler Master Hyper 212+.
Yeah, you could try just raising the bclk while keeping the voltage at stock. However if you raise the bclk make sure to manually set the voltages to default because if left to Auto then the motherboard will raise it for you.