Magic: the Gathering groups its mechanics into five colours, with each one generally considered to have two close/'ally' colours and two further/'enemy' colours. These relationships haven't changed even as other aspects of the design have evolved (though explicit colour mentions in cards are a lot rarer nowadays). In particular, I want to make the case that having Red in its current position of an enemy to Blue doesn't really make sense.
Part 1: Progress and Tradition, Order and Chaos
It's generally said that Red and Blue aren't just enemies, but that their rivalry is more prominent than the other hostile relationships the components have (Blue/Green and Red/White). I don't think this pans out in practice. Mechanically speaking, I'd consider the Blue/Green matchup to be the most prominent by a noticeable margin.
Blue has the most synergy with artifacts and availability of creatures with Flying, two things that Green excels at destroying (one of Green's common keywords is Reach, which is a direct counter to Flying). Blue, for its part, is great at preventing Green's big creatures from attacking and getting around its blockers (Flying even does the latter when not interrupted).
Red and Blue also don't really have any opposition in terms of their philosophies. They form the archetypal mad scientist: Red's emotion and recklessness paired with Blue's love of knowledge and pushing limits. Meanwhile, as the section title alludes to, Blue/Green and White/Red have major aspects that are directly opposed. I've even seen around a refusal to accept fate as something that binds Blue and Black together, when I can't see Red liking having status imposed on it either.
Part 2: Wizards cast Fireball
Mechanically, there's a lot of overlap between Red and Blue, something that otherwise often occurs with ally colour pairs. Red's focus on speed pairs directly with Blue's focus on time, giving Red limited access to a lot of Blue's toolkit: temporary copies and control changes while Blue gets permanent access, extra combat phases while Blue gets full extra turns, discarding before drawing while Blue draws directly.
Meanwhile, one of Red's biggest mechanics feels like it's artificially limited from being available in Blue. Blue disregards the board state in order to win from the hand, and as a result has amassed an appreciable number of unusual win conditions to avoid the standard route of attacking with creatures. Burn would logically be included in that sphere, at least more fitting than having the card draw colour want to delete its draw pile. Even when trying to limit this aspect, the overlap still bleeds through. Psionic Blast and Prodigal Sorcerer are standard damage in Blue, just because that's what Intelligence-based characters get up to in fantasy. Meanwhile, Yuki-Onna throws the idea of water and ice being Blue-aligned because it deals damage, darn it!
Part 3: friendship ended with Black
There's one more mechanical niche that feels Blue-adjacent: rituals (spells that directly give mana). Blue itself doesn't use them since it uses its artifact synergy for supplemental mana instead, but that effect is still how to get a fundamental resource from the hand while it's traditionally from the board. This effect was strongly Black-aligned, but it's been shifted to Red over time. To me, this is a result of mana burn being removed. When you took damage for having unspent mana, it fit in with Black often having to deal with self-imposed negative effects. Without it, the only downside of not spending the mana immediately is not having it later. As such, it becomes aligned with Red's speed focus. But it being Blue-adjacent hasn't changed.
Part 4: Loose ends
The other side of Red and Blue being allies is that Red/Green and Blue/Black become enemy pairs while Black/Green is an ally pair. Red/Green rivalry also feels pretty well-supported, pitting Red's speed and land destruction against Green building up tons of lands to fuel massive spells. Meanwhile, the increasing amount of stuff you can do with land cards brings that same ramp closer to Black's ability to search their deck for a silver bullet. Blue/Black doesn't have as much against them, I admit, but I suppose once again one of the colours needs to be without an archrival.
