I would probably try for a low-poke to keep them in check, but I myself am still learning how to fight against Skarlet, so I am not sure.
I do wonder, though...since you can't do anything about the slide after blocking it and Skarlet's other mix-up option off the Red Dash is her Down Slash, which can lead into a full combo and a reset...why would you bother trying to block the slide? If you're hit by it, you only take...8% damage, right? Then you have a chance to time a Wake-Up and break her pressure control, or try and back away from her. When you think of it like that, it may not be worth trying to block the slide.
However, it can be argued that a player wants to avoid taking damage as much as he can, and the slide can begin to become a problem when you're running out of health. It's that psychological setback that sometimes makes them block low.