The Laws of Prayer (The morning services) (10)

1. A person may not mention the Name of Hashem whether in blessings or prayers when his head is uncovered. It does not suffice to cover the head with one’s own hand as the hand and head are all part of the same body, and a body cannot cover itself. One may, however, allow someone else to cover one’s head with his hand to make a blessing. (Shulchan Aruch and Mishne Berurah 90)

2. A person may also pull his sleeve over his hand and use the sleeve to cove his head while reciting the blessing. (ibid)