Maybe they were using leftover ANH sabers for ESB and leftover ESB sabers for Jedi?
Well, the ANH lightsaber was a feature built into the figure, whereas the ESB saber was an accessory. Also there was never a blue ESB lightsaber, they were all yellow.
I’m guessing that when they finally decided to fix the yellow lightsaber thing and give Luke a properly coloured saber they based the new lightsaber accessory on the early ROTJ footage in which it was blue. But, like I mentioned earlier, they did change it to green eventually.
