How do you pronounce a D at the end of a word followed by a word starting with D?

This is less a matter of pronunciation than diction.

Diction is defined as:

the accent, inflection, intonation, and speech-sound quality manifested by an individual speaker, usually judged in terms of prevailing standards of acceptability; enunciation.

Unfortunately, there is going to be a considerable amount of variation from person to person on this matter. British speakers will enunciate and elide differently from Americans, and so on and so forth. Regional accents will also impact heavily upon this.

As Robusto points out, the double d is usually only separated to clarify one word from another by most native speakers. But, someone whose profession requires more elocution (announcers, audiobook narrators, radio hosts, etc.) might do so as a matter of course. The British announcer in the linked video is deliberately extending his word endings in what we typically refer to as "announcer voice".

I have an American accent, I'm from the Northeastern United States. I'd say red door as two d sounds. But, I tend to elide blind date into "blinedate". In this case, it has more to do with frequency the words go together. Blind date is more common than red door, and hence may have melded into one word for me.