Hi. Sequematic doesn't integrate directly with any hardware, but can be used easily with IFTTT to connect with any of their supported devices.
In terms of hooking in to Dark Sky, you should be able to use the custom webhook call step to make a call to their API, and you could then store the response in variables. If you follow it up with a variable condition you could then direct the flow of the sequence based on the API response.
There's more information about the custom webhook call here - https://sequematic.com/help/step-custom-webhook-call
As you're building the sequence, if you need help just post back here and I can provide assistance.
Another option would be to use the weather applet in IFTTT to trigger your Sequematic sequences.