I was able to get some feedback on types of blue line and the process itself from the great Sean Parsons! He was open to me sharing his process so here it is. ( I will add that he does have pages currently for sale and can be reached via Facebook )
When I asked why sometimes it is clearly visible that ink was over a blue line and other time it was difficult to tell he gave the below response.
Some of it is the shade of blue people use. I've got original inks of mine over blueline that I won't even try to sell because I picked a color of blue that was too bright and I didn't do enough clean-up of the art before I printed it out. Sometimes a penciler either draws with a crazy light and you have to darken the scan to see the pencils, or they draw super dark and with a soft lead that leaves smudges all over the page. if you don't take that into photoshop or some other program and clean it up, stuff you don't even notice on a scan will ruin a printout. In some terms that might not make sense to a non artist, if you smudge pencil lead by rubbing your hand across a page of pencils which happens constantly as a penciler works, it might leave a light gray smudge on a white background. Say its barely noticable, like 5% gray on white. Barely a shade darker than white. Super light gray now. If you turn that page into blue, and print it out, what was super light gray on white, is now blue. There is no way to not notice that on paper now.
a lot depends on the blue shade you use too. Some blues are light but well visible, but work better on a dark penciled page than a light penciled page. Light pencils read better on a page if they are more of a light mint green shade. Sometimes we will print the pencils out in gray color instead of blue as well. it blends better with the ink and is less obvious than blue printed pencils, but takes more cleanup work when you scan the inks to send to the colorist. I use gray occasionally, because 20 years of inking with my face inches from the page has ruined my eye sight, but I also don't like the gray printed pencils because I think some inkers use gray to sort of trick people into thinking the art is over original pencils. If you see a pinch of gray sticking out, its easy to believe its pencil lead, and if the page isn't stated to be inks on blueline, buyers can be fooled. That's why I try to be sure a buyer knows if he's buying bluelines or original pencils and inks from me.
another factor is paying attention. In photoshop, there is a box that opens that shows you the difference between a color you choose, and what it will look like on screen and another box that shows what that color looks like in print. Those two boxes never look the same. It almost always prints darker. so a nice light blue might look perfect in photoshop, but prints 3 shades darker. most inkers say "oh well" and start inking. I've made that very mistake plenty of times, but paper and printer ink aren't cheap and some guys don't want to go back and spend ten more minutes changing it around and reprinting it again
a page that looks this color in photoshop...
might print this dark when you print the page out...
this dark blue page is a great example of what I was saying about smudged pencil lead ruining a printed page too. It looks fine as you see it here to my eye. But, looking at a close up of the gutter of the page, that is the part of the page between the panels, you can see that what is supposed to be white has smudged lead on it, and it became a blue smudge when the page was turned into blueline. No one took the time to clean it up. When I clean up a small section, clearing up that smudge, you can see how white it should have been. Now imagine holding that inked page in hand, and how awful that blue everywhere must have made that page.
it should have been cleaned up, and it could have been cleaned up and the blue removed enough to not ruin the original art, but at the same time, cleaning it up also makes it harder to make out certain details. So you have to be a good inker not to ruin the art once some of the pencilers details become harder to read
I have saved a bunch of preset shades of blue that I choose from with each new job. Some shades work better on certain papers, some work better with certain pencilers. it all depends on if the penciler draws light or dark. I have a lot more colors than these, but most of them are based on some version of one of these colors, but this is a good reference point to start