It is very understandable both that one might want to see a "fountain of blood" in this image but isn't it more likely that it is a reference to water? And as a fountain, is it referring to forgiveness of sins or of a violent flood to wash away sinful activity from the land?
I notice that the word translated "fount" is literally of the visible parts of a vagina but that it figuratively means a "spring" of water or of menstrual blood. As such it is also used to refer to one's ancestry and one's descendants.
So the word has some versatility. However, the thought that menstrual blood would cleanse from sin is a non-starter. I also notice this parallel:
KJV Joel 3:18 And it shall come to pass in that day, that the mountains shall drop down new wine, and the hills shall flow with milk, and all the rivers of Judah shall flow with waters, and a fountain shall come forth of the house of the LORD, and shall water the valley of Shittim.
Zechariah says that the fount is "for sin and uncleanness". This might refer to forgiveness, violent removal of all that offends or to mikveh.
YHVH is described as a fountain of life:
KJV Jer 17:13 O LORD, the hope of Israel, all that forsake thee shall be ashamed, and they that depart from me shall be written in the earth, because they have forsaken the LORD, the fountain of living waters.
In Revelation God Almighty says that he will give the thirsty to drink from the fountain of the water of life:
[Rev 21:5-6 KJV] 5 And he that sat upon the throne said, Behold, I make all things new. And he said unto me, Write: for these words are true and faithful. 6 And he said unto me, It is done. I am Alpha and Omega, the beginning and the end. I will give unto him that is athirst of the fountain of the water of life freely.
In John 4 Jesus contrasts the water he gives (the call to the kingdom?) with the water from Jacob's well (perhaps a symbol of the old covenant?) saying the water he gives will be "within you springing up to everlasting life".
Finally, might John have been pondering Zech 13:1 when he made this report?:
KJV 1 John 5:6 This is he that came by water and blood, even Jesus Christ; not by water only, but by water and blood. And it is the Spirit that beareth witness, because the Spirit is truth.
So would it be safe to say that the fountain of Zechariah is not about a fountain of blood nor about floods to remove the wicked but rather about a fountain of water and that the image of water is that of the message of the new covenant that brings life?