Thanks to everyone for your suggestions. I had already glued the base on and sanding wasn't an option, so I followed the suggestions given, cut up three small pieces of the same wood, gave them a coat of 50/50 BLO & MS, and then tried three different colored stains on them, over the BLO. This worked, so I just applied a new darker stain over the BLO and it looks fine.
I know poplar isn't a favorite wood around here, but I had a 6' piece and had to use it SOMEWHERE! You know, "Waste not, want not."