I find that Blue Dot burns cleanest at close to near maximum charges. You have to realize though that Blue Dot is primarily designed to be a shotgun powder though, it also seems to work better in longer barrels, than in shorter ones.
In my experience lighter charges of Blue Dot aren't as clean as heavier ones, but most bullet lubricants aren't exactly what I would call clean burning either.
I don't find Blue Dot to be very dirty. If a starting load isn't as clean as you were expecting, don't fret it, and keep working with the caliber, as you get closer to maximum loads it cleans up quite a bit.
As you can tell, I like Blue Dot, and as we get older, we all tend to get just a little a bit opinionated.
Blue Dot is a somewhat bulky powder, so loads will be near max case capacity. I think this is a bit safer, and burn rates along with accuracy seem to be more consistent because of this.
You will not accidentally double charge a 9mm shell with Blue Dot, without the powder spilling out.
I will also say that when using Blue Dot, I have never had to dump a powder measure, and throw it out because I wasn't 100% sure what was in it.
The unique blue colored identification flakes that give the powder its name are actually pretty neat. If someone else was using the machine, and left the powder hopper full.... well better safe than sorry, and I really hate to have to throw any powder away.
All that being said, Blue Dot is very good at what it does, and is a very useful tool in your re-loading toolbox.
There is a good reason that people use Blue Dot, and it's name keeps coming up. Quite a few people have found it a convenient powder to achieve major velocities with in 9mm, without showing signs of excess pressure. It also seems to work good with gas sealing particularly troublesome cast boolits. I think you should give it a shot, it might just do what you are looking for.
I think it metering well all depends on what type, and or brand of powder measure you are using. I have had good results with it in a RCBS Uniflow, it might not meter well in say a Lee Auto-disk, but you would have to ask someone who has used it, in your brand of powder measure, or just try it out yourself to see.
In the end you will be the final judge on how well it works for you, and what is an acceptable level burnt, or unburnt powder.
If I place a white towel down on the shooting bench when shooting ball or spherical powder, I tend to notice quite a bit of smaller pieces of unburnt powder that I ordinarily would not have seen, or I may have possibly mistaken for soot or fouling.
An unburnt piece of flake powder however is much easier to spot, but a couple of pieces of unburnt flake, or ball powder don't really hurt anything, or cause any malfunctions.
If a powder is a little dirty, or leaves unburnt flakes, but it is the only powder that keeps my gun from leading, or lets me achieve the velocity that I am looking for, I can live with a little bit of unburnt powder. I will just clean the gun after I shoot it.
- Bullwolf