The baby gurgled quietly, snuggling into its blanket as it slept.
 
No, its a girl, not an it. Blueblood thought, staring down at the little thing.
 
After many months carrying her, here she was. Months of lies and evasion from notice and now he had a daughter. She was so small though. Smaller than he had expected. Surely, babies weren’t supposed to be so tiny, right? But who could he ask?
 
The filly let out a small sneeze, barely a whisper over the fireplaces crackling flame. The duke stared down at her indifferently, looking away as he placed another blanket over her. Babies got cold, did they? He thought.
 
For a moment, he contemplated what would happen if he walked out of this room with this little bundle of fur and yawns. He could go straight to his Aunt Luna, tell her the truth and claim this child as his own. He knew the mare well enough to know that outside an initial scolding and smack to the head, she would support him. His aunt was old fashioned but understanding.
 
He could be the fillys father.
 
But then there would be questions about her parentage. Who was her mother? Why had no one said anything about this? Where did she come from?
 
Where had the duke been for the last few months?
 
Blueblood’s hoof landed on his stomach, where just hours before, his daughter rested in. When he’d still been a mare… How could he admit to such a thing? That his stupid escapades had resulted in a child and now he was claiming her. That he, even now, had no idea who her parent was.
 
He knew what would happen. The sly looks and hidden snickers. They would mock him and call him terrible names. He could live with such a thing but what about her? Could she? Could he raise her to be strong and noble yet still admit to his own wrongs? To her, he would be a whore, something used and disgraced by anypony who chose to look.
 
She would be ashamed to be his daughter, would she?
 
So he steeled his heart and bundled her up, his plan going just as he had intended. She would have a much better life without him.
 
*****
 
So, now Crescent’s ‘mother’ has been revealed, though I feel it was a bit obvious.
 
Blueblood has always been an adventurous pony, doing things others would never think due to his connection with the Diarchy of Equestria. As such, he frequently had affairs thought mostly with mares due to Canterlot being not so welcoming of same sex unions. They could tolerate Celestia and Twilight since they were alicorns and above them but regular ponies? There was no way.
 
This led to Blueblood hiding his own interest in stallions by sleeping with the guard ponies around the castle. This was a well known ‘secret’ that led up to the duke using a sex change spell to ‘liven things up’ and resulting in an accidental pregnancy. This forced the duke into hiding as the spell kept him from changing back into a stallion until he gave birth. From the beginning, he intended to give he child up and pretend it never happened.
 
Crescent Moonstone’s mother never wanted her. From the day she was conceived to the day of her birth, her mother wanted nothing to do with her. That’s why she was given up at birth. Yet despite that ponies’ intention, Crescent was still their child and couldn’t help but adore her.
 
With a heavy heart, Blueblood gave her away, hoping she would just end up in foster care somewhere. He had heard of ponies from far away who were willing to take noble ponies unwanted children away with no strings attached but had no clue what they did with them. Had he known, he might have thought better than to give her up.
 
Later on, when Blueblood saw how Sirius was treated by the Canterlot elite and how their words affected the dolt, he felt somewhat relieved he’d saved his daughter from such a fate. Still, as an uncle, he felt it right to advise the young prince how to properly deal with such ponies.
 
 
TLDR: Blueblood had lots of fun with stallions, got preggers and gave his kid up to some shady folks cause he didn’t do enough research.