- GM sets a price in gold pieces for one week of training, player rolls a d20 or 3d6 Intelligence check; success means ability is gained, failure means the character is still studying;
- Player notes that a character is studying an ability; when the character gains enough experience to gain a level, the player can opt to add one studied ability instead, resetting experience points to the start of the current level.
A few abilities would eliminate risks from a specific kind of action, like the blind-fighting example, or provide a bonus on a specific kind of roll, like a Stealth ability (hide in shadows or move silently) that adds 1 to the chance to surprise. None of the "bonus" abilities would stack; there wouldn't be levels of Stealth, for example.
I would work up a short list of likely abilities, but most could be improvised with these guidelines as needed. A player wants an ability to catch thrown weapons in flight? Fine, just figure out what the risk would be for an ordinary character to do that, then let the "spear-snatch" ability eliminate that risk. It's up to the PCs to find someone or some way to help them train, and up to the GM to decide how much it will cost, or if there are risks during training.