If these women were prostitutes, as purported, the blood and semen could've come from a customer, not necessarily the killer.
That was my first thought, as they all were just that. I still believe the book Jack the Ripper: The Final Solution nailed it - a successful Victorian Watergate coverup if you will.
I thought the title was presumptuous and read it with a skeptical eye. However, the chapter titled "All Roads Lead to Dorset Street" made me think. The author points out that in 1867, there 80,000 prostitutes out of three million in London (probably more in 1888), and out of all of them, the Ripper killed five who all lived less than 300 yards of each other. The fact that they were scattered about made people think they lived in the area they were murdered in.
Basically, four prostitutes got wind of a Royal Indiscretion, tried to blackmail the government, and got bumped off by a crew of three (he identifies all of 'em). The fifth was murdered by accident as she had the same name as one of the targets.
That book made more sense than any of the other "single guy did it" versions.
From what I’m reading the blood was from the prostitute, so not the killer’s. They tested the semen and like you said could have come from a customer. Agree. People back then didn’t wash their clothing often so the semen could have been deposited on the scarf weeks, months or years before her death.