Coticules and Belgian Blues both use garnets to cut. The blue's garnets are between 10 and 20 microns and the Coticule has garnets ranging between 5 and 15 microns. Hence the Blue has bigger garnets, but still it is a much (and I mean much) slower hone than the average Coticule. Garnets are a sort of faceted round particles that spin underneath the steel while cutting. I think the bigger garnets of the blue have the pressure spread out over a bigger surface and therefor penetrate the steel less deep, making it a shalower cutter than a Coticule. Maybe the garnet's faceted edges in the Blue are not as keen as those in Coticule. I'm only speculating. But the everyday experience is that with many Coticules you can remove steel pretty fast, as long as you raise a slurry. It doesn't need to be thick. Milky is fine. On hard razor's steel (and also on other tool steel), the Blue with slurry comes nowhere near the speed of a Coticule. Actually, the funny thing is, the Blue is mined for as long as Coticule is mined (many ages), because they can't get to the Coticule if they don't remove the Blue at first. But it is only very recently that it was discovered that the Blue has honing qualities. The Blue is a great polisher of already keen edges, and it has also the ability to refine an edge after it comes off the Coticule with slurry. The use of slurry makes the keenness of the edge level off at a given point. But the Blue with slurry is able to push it further than the Coticule with slurry.
Although I own a DMT1200, and used it at the time I wrote that article that's now in the Wiki, nowadays I hardly ever use my DMT. I always set the bevel on a Coticule. If the slurry shows gray discoloration within 15 to 20 laps or so (or even sooner), than your Coticule is fast enough to do bevel work. The resulting bevels are excellent: they shave arm hair quite well and have no deep scratches, which leaves the blade completely ready for further keenification on the Blue, or on any synthetic hone in the 8K-10K range. Some people even take it (progressively) all the way to a Shapton 16K before finishing it on the same Coticule, now with only water. Coticules are extremely slow when used with water only and they leave an edge that tends to be very forgiving and smooth during the shave, for many people.
Best regards,
Bart.