Mile marker, rest area, and restaurant signage has been more or less universally blue and white for ages with no visibility issues. I'm sure it'll be fine. Additionally, I would think that having the blue a bit darker would actually enhance color contrast in poor conditions even if it detracts from the initial eye-catch.
Most directional signage is actually green, per the Manual for Uniform Traffic Control Devices (though street name signs often vary by jurisdiction, with blue and brown also being relatively common).
Although there are a handful of exceptions, each color represents a different type of signage with a different purpose. While drivers may not consciously notice the entire color pattern, most are familiar enough to be able to quickly decipher whether they need to read a sign at a particular moment, without actually having to read it.
- Green: Wayfinding & directional signage
- Blue: Expressway services (gas stations, restaurants, attractions, etc.) & Accessibility-related signage
- White: Regulatory (speed limits, HOV restrictions, parking restrictions, etc.)
- Yellow: Warning (tight curves, steep grades, animal crossings, etc.)
- Orange: Temporary traffic control (typically construction-related)
- Brown: Recreation
- Lime green: Schools & pedestrian crossings
- Red: "Stop" & "Do not enter"
- Purple: Typically associated with toll roads (though not exclusively)
While jurisdictions and private landowners are allowed to create their own standards, these are generally what is accepted and recognized throughout the US. If and when different designs are created, they should be done purposefully, and with consideration to legibility, clarity, and simplicity. WDW's old(?) signs did this very well, using a bight colors and bold fonts that could be easily read from a distance, and distinguished them from the public roads. Prior to that, most of WDW's signs had been brown, similar to the MUTCD standard for recreation purposes.
The two different shades of blue on the new sign are perplexing, as they're too similar to one another to really stand out. The arrows also seem difficult to read; they appear to be based on the MUTCD's "down" arrow that's typically used to designate specific lanes from an overhead sign, but are being used as directional arrows to point outward. The fat arrow and narrow tail just don't read clearly in this context. The shapes and colors remind me more of a webpage or mobile app design than something that would be used for a physical sign.
The design of these signs is odd: they're unique enough that they don't comply with the MUTCD, but similar enough that they don't really "feel" any different. They also aren't as clear and easy to read as either the standard signs or the old ones. This seems to be a great example of how something isn't necessarily better just because it's new and/or custom-designed; sometimes existing elements or standard features actually work better.