I doubt they'd blacklist the phone. Its not like youre stealing it or performing insurance fraud or anything like that. If I were signing up for a new sprint account and purchased a phone at a new customer pricing, and then decided to cancell my contract 2 months down the line, would they then blacklist the phone I purchased fair and square? Of course not. They would just hit me an ETF. A Sprint phone with an acceptable or 'clean' ESN is more valueable to Sprint than a blacklisted one, since it has the potential for attracting new customers and/or the return of old customers.
If your BF is willing to pay the ETF, id say go for it. Save your upgrade points for the device you really want. If i were in your shoes, I would probably do the same thing. But that is just me...
