Camped up here last year, absolutely phenomenal! The hike can be a bit grueling and since I've done it the road leading to the trailhead has been shut down, so plan on adding a few more miles to your hike to get to the lake (around 7 miles.)
The first few miles on the trail are fairly level. Once you make it to Olympic hot springs (which is an amazing place to stay in it own right!) Take a right an start climbing. It's pretty much all uphill from there for 3-4 miles, but it's so worth it.
Boulder Lake is amazing and almost nobody camps up there it seems. Find a nice spot along the lake and take a dio in the lake as a reward for making it all this way!