Today is the first day Tolkien Week, which is the week containing Hobbit Day (Sep 22, the birthday shared by Bilbo and Frodo). To kick it off I'll discuss the Balrogs in the Blue Book.
The Blue Book contains two references to Balrogs, each on page 14. These refer to the original OD&D text rather than being additions by Holmes. The first mentions that they are highly resistant to magic, and is a reference to the original entry for Balrogs in Monsters & Treasure (pg 14). The second reference describes Balrogs as being able to shatter Hold Portal spells. This is from the original description of the Hold Portal spell in Men & Magic (pg 23), and was probably inspired by the Balrog who broke Gandalf's spell on the door when the Fellowship was in Moria. While the OD&D books were eventually revised to remove these references to Balrogs, they were never removed from the Blue Book.
Suppose you want to use a Balrog in your Holmes Basic or expanded game? I'd suggest looking to the original entry in Monsters & Treasure. A transcription of this entry, as well as the original entry from CHAINMAIL, can be found in this post, courtesy DF member DuBeers. As a bonus, here's a screenshot grabbed from a recent auction photo posted on Acaeum: