You have two instances of Juicebox on your page:
new juicebox({
containerid : 'juicebox-container'
});
... starting at line 49 and:
new juicebox({
containerId : "juicebox-container",
backgroundColor: "#FFFFFF"
});
... starting at line 93.
Both instances are being embedded into the same <div> and the first one is momentarily displayed before the second one replaces it. Because the first one has no backgroundColor explicitly specified, it will use the default color of '#222222' which is what you are seeing as flashing.
Use only one instance and delete the other. If you choose to use the first instance, add the backgroundColor : '#ffffff' parameter to it and leave the following code on your page where the second instance currently is:
<div id="juicebox-content">
<div id="juicebox-container"></div>
</div>