tube_ee
First off, just so everyone will know, I'm not recommending Blue Dot as the powder of choice for .38 sp. because the best application is for +P with loads at or near maximum charges. There are a couple of things that might help though. Since the time of the SPEER #11, SAAMI has lowered the pressure rating of .38 Sp. and .38 Sp. +P. The load you're using is for a 158 gr. LSWC, so the charge would be higher for a JHP based on that alone, but at 6.3 grs. it is a start charge for the 158 gr. SPEER SWC in the #11 Manual. Another thing you'll notice is that powder manufacturers data is a bit more heavy laden with liability concerns. Load manual data will run higher for charges even if they quote the same pressure rating as the powder manufacturers data. By all means, get some load manuals. The Lyman Pistol & Revolver is a good one as is the SPEER.
Blue Dot will do better in .357 Magnum cases because of the higher operating pressure of the cartridge. I've burned plenty of it, so I know why you like it for that. Personally, I have never had any problems with Blue Dot. I have seen reports of pressure spikes, but I was also aware at the time that there were some lots that left the factory that burned faster than typical for the powder that some folks might not have been aware of. I still have Blue Dot on the shelf, but when it comes to higher velocity .357 Magnum loads, I switch over to AA#9, anyway. For the type of loads that you're talking about, I wouldn't worry much about excessive pressure, but you may still see unburned powder granules.
I don't recommend interpolating data. Get a good manual and you won't have a need to. Now, I won't say that I've never done it, but the cases where I have, are pretty isolated and based on many years of experience. The other case has been when there was NO data for a particular powder & cartridge and then I consulted with the powder manufacturers ballistician. In this case it was for using V-V 3N37 with 230 gr. JHPs in .45 ACP, and no data existed.
Blue Dot will not burn well until you get near the max. charges listed for the cartridge and since the pressure rating for the cartridge has been reduced, I'd look at one of the other powder recommendations since you're out. If you shoot these loads in a .357 Magnum revolver, you can probably pick up an older manual like the SPEER #11 at a gunshow. I don't know what type of revolver you're shooting, so in the interest of safety, I don't want to put loads up on the forum that could be used by others that might just be starting out and may not have a revolver rated for them. Sorry, I'm not trying to be evasive, but I'll tell you this: if you don't need top performance of the .38 Sp. cartridge and at +P pressure, Blue Dot is better reserved for your .357 Magnum loads.
I went back and checked the advisory for loads of this type.
WARNING! The following data comes from the SPEER #11 Reloading Manual and is higher in pressure than the current rating for .38 Special +P ammunition.
The load range for SPEER's 158 gr. Lead SWC is 6.3 - 6.9 grs. of Blue Dot. Velocity recorded from a 6" Ruger Security Six is 956 FPS for the maximum charge.
The load range for SPEER's 158 gr. JHP is 7.5 - 8.4 grs. of Blue Dot. Velocity for the max. charge was 1051 FPS. The pressure rating for .38 Sp. +P was 22,400 CUP at that time. In PSI, that would be near 23,000 PSI, and 4500 PSI above the current SAAMI pressure limit of 18,500 PSI.
Walkalong: standard deviations for True Blue are phenomenal from .380 to .454 Casull, typically single digit with several under 5, but you'll need their #3 load guide to get SD numbers. They quit printing them in later editions. Personally, I find it a better alternative to Unique for guys who might want to keep powder inventory simple. It is a slower burner and around HS-6 and V-V N340 in burn rate. I don't know if it's isolated to CZ 75s or not, but my 75 B in .40 S&W loves the stuff! And, with jacketed and hardcast bullets. Really good in .45 ACP loads as well when you need velocity, or a powder that will push 230 gr. bullets to around 900 FPS. It has worked well in 9mm as well as .38 Sp. and it's suitable for medium velocity .357 Magnum.