If a girl donates blood to her boyfriend (or vice versa), how will this affect their offspring in the future?

We exclude in advance all issues related to the transmission of various infections through the blood. Everything is obvious here, it seems to me.

If a girl is a donor for a guy, it will not affect in any way.

But if it is the other way around, then the influence can only be if a number of events coincide:
1) girl - Rh-negative
2) guy - Rh-positive
3) during the transfusion the doctors blunted it
4) the girl is pregnant for the first time with Rh-positive fetus

In this case, the girl antibodies to the Rh factor will have already been developed on the blood of a young person - and the likelihood of developing a Rh conflict is much higher than in the case of the first pregnancy in a patient who has not previously had contact with Rh-positive blood.

In the case of the second and subsequent pregnancies with Rh-positive fetuses - there will not be much difference, because antibodies are already produced during the first pregnancy.

Nothing. Blood transfusion does not affect the DNA with which the sex cells are produced. It's just that for some time the oxygen in the body of the recipient will be carried by foreign red blood cells. Then they will die, but by that time their relatives will replace them.