Only Johan is allowed to promote

Having traveled extensively, there are many places where there are very few black belts and they are dispursed cities apart so, as Joe Rogan so eloquently put it (Paraphrasing:"A Purple Belt is basically a black belt who just has to put in some more time." So from that standpoint I agree, as long as it is a solid purple belt who took more than 6 years to get there. Having a long relationship to the art helps to see it holistically, where as wrestlers winning comps may perform well, but it doesn't always lend itself to being able to pass the skill to others effectively. Jiujitsu is about more than just victory in competition. Just my 2 cents.

I have seen on Instagram today, two people locally to me have been promoted to blue belt by a purple belt. Is this allowed? I seem to remember it being allowed for remote locations discretionally if there are no black belts around but there are dozens of Black Belts locally to the area.

I've always known two under is old school. Blue belt can't give stripes to whites. Purple can. Brown can promote blue. Now a black belt can promote any colour but you need to be 2 stripes to promote black.

I have never heard this. I always heard you could promote to one belt below you. This is what the V1 of the IBJJF Graduation System documentation said too. "In countries or regions where there are not enough black belts for the development of the sport, IBJJF will accept purple belt and brown belt athletes to sign as instructors. Once the minimum number is reached, the use of instructor status will be suspended. Brown belt instructors can only graduate athletes to purple belt and purple belt instructors can only graduate athletes to blue belt."

What if a brown belt has blessings from a purple belt, can the brown belt promote someone to blue then?

Only if that purple belt lives in a country that doesn't have enough black belts :)