Ok cool, I've been doing that exactly. But since blue engrams have a small chance to drop purples, should I just hoard all of those as well?
I wouldn't worry to much about blues/greens until your maybe around 35-36 then if your equipment is decent enough you might start hoarding a few engrams until 40 but blues are easy enough to farm

I took a screenshot of when I just dinged Rank 1 with Gunsmith. Same thing, it says Legendary weapon reward, but gave me coins and motes instead!