I am unable to provide a complete refutation of the claim, but use of the word 蓝 to describe blue is not a vernacular invention.

When I first read your claim, I thought of the common turn of phrase “青出于蓝胜于蓝”. 《中华词源》 by 雅瑟 provides the following passage, which I will transcribe here (as it cannot be pasted): (emphasis mine)

此语源于南北朝时期。... (i have elided the rest of the passage) 人们做了一首歌谣：青成蓝，蓝谢青; 师何常，在明经。

Basically, the book is saying that the phrase originates in the Northern Dynasties Era, where both the words 青 and 蓝 are used in a phrase which has the rough meaning of "the student has surpassed the teacher".

The 南北朝时期， or Northern Dynasties era, was from 420 - 589 AD, which predates the arrival of the vernacular by at least one thousand years. If so, then the claim is bunk.

One possible objection is that the use of 蓝 here might not refer to the colour blue, but rather something else. However, the book also raises an interesting point regarding this: (emphasis mine)

为什么用青比学生，蓝比老师呢？ 它的来源是荀子著名篇章《劝学》中的一句话： ”学不可以已。青取之于蓝，而青于蓝；冰，水为之，而寒与水。” 这句话用来教导弟子学习永远没有止境，希望弟子们能够坚持不懈地学习，后代一定超过前辈。

Here is my shitty translation for the benefit of those not fluent in Chinese:

Why use green to depict the student, and blue to depict the teacher? The origin comes from one sentence in [some guy]'s famous essay "An exhortation to study hard": "Never stop learning. [The dye color] blue is derived from the indigo [plant], but the dye's color is more vibrant than the plant's; ice comes from water, but is colder than it." The sentence is meant to teach his disciples that there are no limits to knowledge, and hopes that his disciples will never stop learning, and supersede their predecessors.

Here, the dye is called 靛青, or 青 for short, and you can see a nice photo of it here. The indigo plant is 蓼蓝, or 蓝 for short. To recap: 青 refers to the dye, and 蓝 the plant.

Although this seems like it could lend some credence to OP's original claim - that 蓝 is used exclusively for the plant, and 青 for the rich blue colour, I find this unlikely. Particularly in the olden times, the word 蓝 would probably have already been used to describe the colour, just like the English word 'rose' describes both a plant and a colour.

tldr: the word 蓝 was used over a thousand years ago to describe a plant that gave blue dye, if not the colour itself.

This might be a question better suited to /r/linguistics