1) Fantastic pic. Given a pic like that, I'd say you already saying that you are willing and open to become Black Impregnated.
2) Conception is a HEAVY subject. The correct time is when every party to this potential is ready and the groundwork has been laid. At least one of the 3 of you will have a freekout, usually.
Have you measured and accepted all of the consequences that an interracial baby will bring? Has the Hubby been consulted, prepared and given his consent to be the surogate father to an interracial baby? Is the Bull mature enough to accept the roll of Sperm Doner, and then walk away? Or, is the Bull ready to accept the responsibility of being 'there' for the baby as it grows and matures? If you can answer 'yes' to those questions, they you are ready.
3) But, to just 'bring up the subject', a light conversation is good to set the ground work.
Examples: During sex, whisper or shout - Breed me. (the after sex conversation will be interesting) After sex, ask your lover about carrying his baby. During the first part of your date, talk about contraception and the consequences of pregnancy. Ask your Bull if he has breed any females in the past or what his thoughts are on the subject. That will get things started.
4) The Big Moment in this will come when your Bull takes you into the bathroom for you to flush your birthcontrol pills down the toilet. After that, it will be just a mater of time.