NB: this question has had some extensive editing. I apologize to those who replied early based on unclear premise.
TL; DR two societies (druids & nomads) each discovered one kind of mana (resp. blue & orange) and never suspected the existence of the other kind (resp. orange & blue), even though both kinds are interacting with their environment. I am looking for the simplest explanation for that.
In my story, magic operates through a mana field. Mana comes in two varieties: blue and orange. Flows of mana of either kind are subject to elemental influences: mana streams are naturally found along water or wind streams, trapped into some geological sediments and expelled with lava flows. All over the world, evolution has driven a few animals and plants to take advantage of both kind of mana.
Humans however, like most living species, are not normally able to perceive the presence of mana. Some druidic society, living in forest areas, has discovered a special berry. When consumed it temporarily modifies the vision, allowing to see flows of blue mana. The way this works is by making blue cones sensitive to blue mana rather than normal blue light, hence dense concentrations of mana appear blue to those who consumed the berry. They have prospered thank to the use of magic, but never discovered the existence of orange mana, even far into medieval times, long after the existence and usage of magic has been well established, studied and theorized.
On the contrary, a society of nomads living in an arid desert has discovered independently the use of orange mana, by consuming the venom of some snake. The effect on vision is similar but affects orange cones, which among humans only some women possess. The venom also affects green cones but to a lesser extent. Therefore this society has mostly female mages and has grown to be matriarchal as a result. For the purpose of the question, let's say that around 30% of women are born with this additional cone.
At some point, descendants of these nomads became sedentary and established cities in the desert. They also theorized magic and have a good grasp of it, though not aware of the advances of the druidic societies.
For my story, it is very important that at some point in history, a cultural clash between these two civilizations be rooted in their completely different handling of magic.
- Both societies knew early on about magic ;
- Both societies had long-lasting scientific tradition of describing the fauna and flora around them ;
- Both kind of mana are pervasive to the world and necessary for a complete understanding of the species that use them;
I expect that if one waits long enough, both societies are bound to find out about both kind of manas. After centuries, they still did not.
What is the simplest possible explanation (i.e. as little new elements as possible) that prevented or delayed these discoveries until late medieval times?
Some limitations I'd like to emphasize:
Both kind of mana are (roughly) equally available in both regions.
Apart from druids and nomads, other societies have discovered magic too, but never both types.
Although both the berry and the snake are endemic of the said forest and desert, respectively, they are just two examples of species that found a use in becoming sensitive to mana. There are many non-endemic species that use either kind of mana as part of their metabolism.
Your answer may or may not involve the existence of other ways to become sensitive to mana.
The nomads and druids descendants have lived separately before the above-mentioned clash. They know of each others through tales of itinerant merchants, but they never suspected that their magics were so deeply different.
More Background Info
The following is not necessary to answer the question, but provides background for the curious.
- Mana in itself is actually colorless, the names only refer to their effect on human vision.
- In my world, the discovery of the berry is the root of druidism, as druids maintained their privileges in early human groups by consuming the berry and using mana to become valuable members of the group, including communicating with other beasts that consume the berry.
- The reason the berry has this effect is a dissemination strategy: the berry traps mana so that crows, who consume the berry in their normal diet, can spot the berry easily and spread seeds throughout the forest.
- The reason the snake's venom has this effect is part of their mating behavior: males harbour beautiful patterns obtained by fixing orange mana to their scales. The pattern only reveals to females that they have bitten as part of the courtship ritual, but not to potential predators, insensitive to orange mana.
- both societies have been in contact with dragons, which are among the rare creatures naturally sensitive to both kinds of mana. They are far more intelligent than humans but do not normally bother with learning how to communicate with their delicious preys.
Final edit: why did I chose this answer?
Since there were several very good answers I guess I should clarify this. In my view Cort Ammon's answer involves the smallest set of additional ingredients: it does not involve either society to be particularly narrow minded, but simply ideological reinforcement traps, which are certainly a real thing. In the same category, IT Alex's and Ister's answers were strong contenders.
I really liked the ripple/repelling/void magic sort of ideas, but they set big constraints on the future developments of how magic works in my world. Answers based on interaction between the venom and the berry or other biological stuff were also great, but I maintain that they fail to explain why druids (resp. nomads) were not worried about other species.