How do I attach the back of sheets of paper when layflat binding a book? I want to attempt to bind some booklets myself. An issue is that the paper I will be using (darkroom paper, to be exact), has only one printable side. Therefore, were I to bind the booklets the regular way, I would end up with spreads of which only a single page would have anything printed on it. 

To tackle this issue, I decided to do layflat binding. This way an entire spread can be printed. However, this also mean other spreads will have two unprintable pages. For this reason I intend to somehow attach the pages of these empty spreads to each other; whether with some kind of adhesive or perhaps by sewing them.

I am worried that using an adhesive will create an uneven surface, as I think of some experiences with Pritt sticks and regular 80 gsm paper (I must add, this paper is thicker at 190 gsm). Sewing, on the other hand, may not look very clean as I have very little experience with this method- and none with paper.

Therefore, are there any 'clean' methods you can recommend me?
I don't know if there's a "standard" glue for this, but I would try rubber cement (regular solvent-based type, not water-based).  It doesn't warp the paper like water-based glues, and doesn't leave chunks like glue sticks can do.  The layer dries to a small fraction of the thickness applied, so variations in application thickness aren't visible.  With two sheets of heavy paper glued together, it won't be brittle when pages bend in handling.

Brushing it on the normal way holds paper well, but the paper can be easily separated.  There's a stronger way to apply it.  Brush a thin coat on both sheets and let it dry without joining the sheets.  Then brush another, thicker coat onto one of the sheets and while it is still wet, carefully align and join the sheets starting at one edge.  If you join them while the glue is still wet, you may have a brief ability to adjust the alignment a little if it's off.  If you wait for the second layer to dry before joining the sheets, it will be like contact cement -- the alignment will remain the way you joined them.  

It wouldn't hurt to immediately press the sandwich between two flat surfaces without shifting them (it doesn't need much pressure, and just a couple of minutes; putting the page in a cardstock folder like used for hot lamination will help avoid accidentally shifting one sheet when you press them).  Give the page maybe an hour for all of the rubber cement to thoroughly dry before working with it.  If any excess is pushed out, it will easily rub off when dry.

This "two coat" method gives a pretty secure, permanent bond.  The sheets won't peel apart in normal handling, especially if they're well-aligned so one sheet doesn't have an exposed edge to catch.  Current formulations of rubber cement are acid-free and have archival life.