Transferring a balance onto a credit card will affect your "leverage ratio" (total credit balance over total credit limit) if the balance isn't coming from other cards. So, if the amount you owe isn't already on your credit report somewhere, or it's in a "non-revolving" credit account (like a balance owed to a utility, or on a structured note like a car payment or student loan), your leverage will increase and that could lower your score.
In addition, opening a credit card for the purpose of transferring a balance will reduce the average age of your credit accounts (ding), and if you close a credit card account from which you're transferring a balance, you will further reduce the average age and also the maximum age of your cards (ding and ding).
Working against those possible dings, regular, on-time payments to the card balance will slowly improve your credit history; derogatory marks already on your credit will get older (meaning they impact your score less) and eventually drop off. If the balance transfer approves your ability to make such timely, significant payments to your debt, it will be a net gain as long as you avoid the gotchas about how credit leverage and account age affect your score.