Actually we can probably get a lot out of them while giving little away if we play our cards right.
First
Upgrading them to the high end of the high end imperial tech, and getting them high end imperial economic tech is likely to go down well (so get them to the point we were before the end sans what ever extra DAoT we had). We can probably buy quite a lot with that hopefully snag us the Terminator and forges in the best case scenario.
Then we can exchange certain bits of Dark Age tech that are shiny, but expensive like conversion beamers (no production tech).
Finally and hear me out here, but I want to sell them our Neo astropath tech.
First because they're system at the moment probably relies a lot on Librarians who should be else where, and a good deal of trust and fudalism, so neo astropaths should help in that regard. Second while neo astropath's are great they're not uber powerful, but they are better than any alternative so we can probably pump them for a lot.
@Durin
1. Have they indicated that they would be open to a tech trade in the nearish future.
Later could mean later later far future or soonish we're just finishing up the initial greetings now lets screw each other for shines.
2. On the Blood Angel's psycic powers, is it right to assume we can't use them because of psycic frequencies meaning only BAs can?
3. Do we know of the twin flaws of the blood angels? (IIRC I think the BD don't suffer from one of them...)