Let me begin this section by saying I HATE DARK LOANS. How can something so good be so evil? I'll tell you exactly why.
 Dark Loans explanation
Dark Loans is the sister bank of the Solar Bank. It is run by Doomy. And trust me. She's called Doomy for a reason.
Dark Loans gives you energy from their bank to add onto your energy/battery. Sounds great, right? Wrong. It's a loan, meaning you have to pay back every bit of sol you get. Whether that's the next day, the next week or the next month, you have to pay it back. The manual way they do this is taking sol from your Solar Bank. However, if you don't have enough sol to pay them back with, I'm afraid you are going to get extremely, EXTREMELY annoyed.
 Dark Loans penalty
So you can't pay your sol back, huh? Well there's no escaping Dark Loans. Doomy is a supernatural loan shark and is going to bug the hell out of you for the next 15 minutes of gameplay.
You pay the sol back by running on a form of treadmill; she will ask you to generate electricity. It's a kind of mini-game, except mini-games are fun. This isn't. You run by tapping A and Left in turn. Not so bad? Very bad. When you look at the amount you have to pay back and the average time it takes to pay ONE sol back, you are going to scream. This is a very long and agonising process and is not very pleasant on your thumbs.
On rare occasions, Doomy will let you off when you get to about half-way. This is her being nice. But you STILL have to pay her back, so make sure you put some sol in your bank as soon as you get it.
This mini-game should teach you not to borrow what you can't pay off. xD
 The Dark Card
In Boktai: The Sun is in Your Hand, upon visiting Dark Loans for the first time, you will leave with a Dark Card cleverly tucked inside your pocket. In Boktai 2: Solar Boy Django, Zazie will give the Dark Card to you (which will be kept in your key items).
The Dark Card is a portable Dark Loans. If you use the Dark Card, Doomy will appear before you. You can carry out your transactions here. Another use of the Dark Card is that when you die, the game will ask you to pay some sol to continue. This is payed to - guess who - Dark Loans. If you fail to pay this back, this will be added to the total owed for the penalty.
This is my advice: Use it for death, or for nothing at all.