You didn't answer one little point about which distro to edit the Grub.conf. Is it the existing Blag or on the other distro I install next?
I don't install second grub to the MBR do I?
Do I say no to installing the second Grub anywhere?
Well, it all depends. When you installed distro A, you probably told it to install Grub in MBR, so your system boots. So basically, yo've got this little bit of code sitting in MBR, and in partition that belongs to your distro A, there is a config file in /boot/grub/menu.lst. The bit of code in MBR refers to that config file, and all is peachy.
OK, so now you're installing distro B, and it asks you about installing bootloader... and you've got a choice. You can go ahead and install new bootloader. This will put a new bit of code in MBR, that will refer to /boot/grub/menu.lst of the distro you've just installed, distro B. This one probably doesn't know anything about distro A (well, maybe it detected its presence, but let's assume it didn't). So you have to tell it about distro A, by making appropriate entry in its config file - in other words, in /boot/grub/menu.lst of distro B.
Or, you can tell distro B NOT to install its bootloader. In this case, your old grub is left sitting in MBR, and it is still pointing to menu.lst file in distro A. It doesn't know anything about distro B you've just installed, so you have to tell it, by booting into distro A and changing its menu.lst accordingly.
Actually, you might have a third option: tell distro B to install bootloader, but in root partition rather than in MBR. What does that mean? Well, when the system boots, it will look in MBR, it will find your old Grub (from distro A) in there, and boot accordingly. So why bother installing new grub in root partition? Just because this is an easy way of finding out what entry distro B needs - see what got written in menu.lst of distro B, and simply copy it to menu.lst in distro A! It's just an easy way of making sure you'll have all those "append " and "initrd" and what-not entries your new distro needs to boot properly...