You have account in work. Colleague has information that could help you grow the account. However he refuses to divulge the information unless you give him the account and from this he could personally financially gain. You wont swap account so never get info

No blackmail,

He is not demanding something, not to do something.

He is merely making an offer to do something for you, and you do not trust him or agree to the terms.

Agreed, tell the bum to share the wealth and credit for success or take a hike and mind his own business. For your part though do your due diligence and see if you can improve this account. He found it, if indeed he has an idea, so can you.

No, this is in no way blackmail. Just him trying to get over on you and get something for nothing. Or just talking crap which may be what you're falling for, to get that something for nothing. How is he even obligated to tell you anything in the first place?