You should probably think through the whole step by step process for a compound slide and some of the answers will pop up.
Sooo. The upper of the compound will generally have a flat surface. So it can be checked against a surface plate and scraped to be sure it's a flat plane with good bearing. The bottom of the compound will also have a flat that bears on the top of the crosslide. IF it has a boss that fits into the crosslide it still needs to be planar, just that you need a small surface plate with a recess in the middle. Think of this as a secondary plate that can be verified from the primary surface plate and can then be used to verify the bottom of the compound.
So, fix the bottom of the upper, make sure that the bottom of the lower is good and then you can work the flats of the lower section using the now-flat upper as a reference. When you know the flats on the top of the lower section are flat, then also make sure they're parallel to the underside, using 1-2-3 blocks or parallels and your surface plate. Adjust as necessary for both flatness and parallelism.
This is all just preparatory to working on the dovetails. So now you can use your small straightedge to check and fix the first dovetail surface, the one on the lower section that's closest to the headstock. You'll put bluing on the straightedge and see how it transfers to the dovetail. You don't need to fret about precise angle, just be sure it's a good flat surface. When that's a good surface you can work next on the opposite side. It needs to be flat and also parallel with the first side. Adjust as needed.
When all that is accomplished, you can turn next to the female dovetails in the upper half of the compound. Now we get to "how you blue up the dovetail". Some people make up a small stick with an angle on the end that might be fabric or leather covered to use as a spreader. At the other end of the process scale you can use your fingers to smear on bluing and rub back and forth to get an even coating. There are foam paint rollers designed for inside corners that might work. In any case, you can now use the blued up (and now accurate) compound lower section to work on the dovetails of the upper. And then there's the gib to deal with.
That's a long-winded way of talking about how to blue up a dovetail, but the final product is built from all the procedures before it.