To accept the blackmail would mean to trust a partner who has proved to be dishonest. There is absolute no reason to justify a blackmail acceptance. Therefore that risk can be ignored.
Yeah, that's why blackmail is only a theoretical idea in the real world - right? 
Exactly. You should look into the whole Cryptolocker thing if you believe blackmail is to be ignored.
Ok lets play blackmail:
We do a trade and I (Bob Blackmailer) will not release the fund at the end of the trade, so you would loose 1.1 BTC and I only 0.1 BTC. I send you a message that I only will release it if you send me 0.5 BTC.
Would you accept?
If you accept to pay then you are in an even worse situation: You can lose now 1.6 BTC, I have already won 0.4 BTC. So I will try it again and will blackmail you. Would you be so irrational to pay again.... Then you would be in an even worse situation, and so on... after a few rounds I think everybody has learned that the only way to deal with blackmail is to absolutely reject it!
If you reject right from the beginning, what would happen? The blackmailer will wait a while, maybe repeat to blackmail you, but at some point he will give up. Then he has to choose to lose 0.1 BCT if he never releases the fund or he releases it and get back the 0.1 BTC. A greed blackmailer will release. And its in the nature of a blackmailer to be greedy ;-).
The only real blackmail problem would be in countries where BTC is illegal (russia) and the other party blackmail you with reporting you to the police. Also an undercover agent could be trading and sending you to jail... In those countries you have probably more problems then buying/selling BTC.
If you manage to do the Fiat transfer in an anonymous way (?) then that could be avoided as well. I have not idea how, but I assume in those countries they know ways to do that...