Isn't the whole point of becoming a keyblade master to prove that you are able to resist the darkness?
No. In fact, Eraqus was going to pass Terra even after his display of darkness until Xehanort made him doubt his judgment. Being a master isn't about resisting the darkness, but showing mastery over wielding the Keyblade. The Keyblade can be used for light or darkness or everything in-between.
And no, because let's also rememer that Riku's main source of power both the darkness and light. KH3D even makes it a point to show that he takes the darkness and purifies it into light.
Even as a wielder whose main source of power is darkness and light, Riku ended up being a master.
Even though Terra resisted it in the Mark of Mastery, Xehanort's seduction to the dark side should not have been so easy. Also, if he was skilled enough to handle the situation, why would he need to access the darkness to defeat the orbs of light?
Terra had no reason to not trust Xehanort's teachings. Imagine having a power that you ignore because you are taught it is wrong, and this this renown master who is your father-figure's oldest friend says its fine to use that power. Why would Terra have thought anything otherwise?
There is nothing wrong with using the darkness. The darkness is just a power that can be wielded by anybody for good or bad, but it is just due to the corrosive nature of darkness that it controls and corrupts people more easily than say the light.
The light is not without its issues. Eraqus admits that he was blinded by the light and his dogmatic views, turning him to a "dark" path in trying to kill his own apprentices/children. Pure light can still be just as bad as pure darkness.
How do you know what's grounds for failure? It was Eraqus' test and Eraqus was vehemently against darkness. If Terra was truly ready to be a keyblade master, failing the test and not being named a "master" shouldn't matter to him.
And again, Eraqus was going to pass Terra regardless of that. He only doubted Terra's ability to control his inner darkness when Xehanort implied it. Terra could have done the whole test using his inner darkness with perfect execution and would have still passed because the benchmark isn't alignment but skill. The variable here was Xehanort's manipulation, not Terra's lack of control.
I brought up Aqua's response to the darkness to prove what a true keyblade master reacts to darkness. She didn't succumb to the darkness at all even at her lowest point. Ansem's dark blast is what converted her. Aqua was in a much worse situation and fought off the darkness. Terra got scared of light orbs, then got mad that he failed, and ultimately unleashed Xehanort upon the worlds. Just because Terra was saved and helped at the end of it all doesn't mean he should be rewarded with a promotion to keyblade master. It would have been a slap in the face if Terra got to become a master, while Sora who did almost EVERYTHING got nothing.
Okay this is a whammy of a paragraph so let's go bit by bit:
Aqua's ability to resist and fight off the darkness is not a show of her skill as a master, but just a show of her determination to protect her friends/family. Aqua's resolve is what makes her a master, not her ability to stand strong against the darkness.
Ansem's darkness ball of darkness and despair was the catalyst, yes, but all it did was amplify her inner doubts and fears that we saw in 0.2. This inner darkness already lingered inside of her. Remember that the brightest light casts the darkest shadows. Aqua is no better than Terra because her resistance to the darkness up to that point.
Terra was just as thrown off as Aqua by those light orbs, my dude. He got mad at himself for his inner failings that were only brought to light BECAUSE of Xehanort.
Why are you blaming Terra for Xehanort's cruel plan succeeding? Xehanort did everything with the intention of those things happening. He knew he could manipulate Terra. He would have moved onto Ven or Aqua had Terra failed as a vessel as Vanitas implied.
Terra shouldn't be named a master, not because of what you said but because the title itself is absolutely pointless.
Sora doesn't want to be a master.
Regardless if Terra was suffocating or not, part of being a keyblade master is powering through the circumstances. Terra wasn't able to cut it - end of story. That's like saying it was OK for Riku to open the door to darkness because he was suffocating on Destiny Islands.
Terra's actions throughout BBS definitely reinforced that Terra probably wouldn't have been ready for the responsibility of being a master, sure, but his inability to control his darkness is not what kept him from being a master. The only thing that kept him from being a master was Xehanort. End of story.