In response to "Can you boost a stat that isn't being tested?": "There is nothing stopping you from using Scrapper or any other similar asset during a test of another type (i.e. it doesn't say 'During an agility test' or anything like that). You may use it during a willpower or intellect test if you wish; you simply won't get any benefit from it besides from Dark Horse as you suggest."
Q: If an investigator has Dark Horse in play, and one resource, then fails a skill test by 3, can they play Lucky! and succeed? A: Yes. If you spend your last resource on Lucky! while you have Dark Horse active, you can get +2 to your skill value from Lucky! and essentially +1 from Dark Horse, because Dark Horse is a constant ability. (Rules Question Form, June 2023)
This is quite an interesting card. Note that the triggered ability only requires that you not have resources at that moment; there's no reason you can't use this with Physical Training or Arcane Studies, or even better the permanent Scrapper or Keen Eye. Fire Axe should also work, as you would use the card action, then the free action to convert resources for +. And it's 0XP to boot, so all of the Dunwich investigators can use it, as well as Wendy Adams and Agnes Baker.
I could very easily see this becoming part of my standard kit in many many decks.
This card can be combined with Lucky! and Lucky Dice, if you spend your last ressource on using them, as the game calculates the final skill value in step 5 of the "Skill Test Timing" (Rules Reference).
Assuming that I'm reading the rules correctly, it seems like you are able to draw your card during the upkeep and then decide if you want to take your resource for the turn or not. This allows much more flexibility in planning your upcoming turn.
"In player order, each investigator draws 1 card. Once those cards have been drawn, each investigator gains 1 resource." - Rules Reference, p. 25
The term "during" seems to apply to any moment within the phase and there's no entry in the Rules Reference for the term.
I just tried Dark Horse with a Wendy Adams deck - and it doesn't work, at least not without Scrapper or similar. Wendy's Amulet is pretty awesome, as it lets you play discarded events - so you need some resources for events. However, if you have resources, it's pretty tough to clear them when you want Dark Horse to take effect. Scrapper would be good, as you can pump a test and clear your resources - but without it, I was stuck.
It is a powerful effect, +1 to each stat - but I think this might suit someone like "Ashcan" Pete who maybe can run a little more empty resource-wise.