Yeah, some makers have use a steel that does not hot blue well. A lot of savage/stevens guns are like that too.
Marlin 90 frames take bone charcoal case colors very nicely. I have had one done years ago.

In regards to “value”… one would generally be hard pressed to ever see a 12g model 90 sell for more than $800-900. This one obviously has a LOT of work put into it. But it would mean someone wanting to pay for that over what the gun is. What that amount COULD be, I cannot say.

I appreciate your input very much. I am into it for less than $700, so I will likely come out okay if I decide to sell it.

I was offered an all original nice condition Model 90 a couple weeks ago for $800 or $900 I forget which but it was a 16 gauge and if memory serves 28” barrels .

I paid about $350 for my 16 ga, but it was a "beater", some surface rust on frame and barrels, and a Crack in the forearm that we.glued.

I don't know anything about Marlin 90's and I'm not much on Over and Unders, but I really like this one. What are the chokes? It looks like it might make a heck of a Doubles Trap gun.

I hate to show my ignorance in this, but I do not know. The barrels are not marked, and I don't have a choke gauge. A dime stops short of going through the top barrel by a half inch but will just go through the bottom barrel.

Sounds like modified and full. From memory I think most dimes are .710. So you probably have more than .030 in the top, a full. That is an old-timers trick to see how tight a 12 gauge barrel was.

US dimes are about 0.703" diameter. I'm one of the old timers that use a dime to determine 12 gauge chokes. It still works.

Yes sir that’s the bore gauge I grew up with for a 12 gauge , float a dime and it was full goes inside modified and if goes inside with more room improved cylinder .

I “had” the O/U you needed ! It was a Remington 32TC 30” with 38 thousandths in each barrel I got it from Feliejn a few years back . Seems to me the gun was made in 35 or 36 if memory serves .