I'm building a desktop using oak wood. I used tongue and groove joints to join four planks together, gluing the planks along the joints with Titebond III, but there were some small gaps between the planks because some of the planks weren't perfectly square. I added some wood filler to fill the gaps and let it sit overnight to dry. The filler filled the gaps well, but the excess added some dark spots to the wood that make it look dirty. I'm planning on finishing the wood with oil polyurethane and would like to remove these blemishes, if possible, before applying it.
I've tried waiting to see if it would dry out but haven't noticed a difference after 8 hours. I've also tried sanding with 100, 150, and 220 grit sandpaper but didn't notice a difference either. Is there anything I can do to remove the blemishes?