If it shows up in the PURGE dialog, it's not being used in the drawing, so go ahead and purge it out.
The *Ublocks are anonymous blocks, created in software, so you can normally just ignore them (except it's a reasonable idea to purge them out)
From HELP:
About Anonymous Blocks (AutoLISP)
The block definitions (BLOCK) table in a drawing can contain anonymous blocks (also known as unnamed blocks), that AutoCAD creates to support hatch patterns and associative dimensioning. The entmake function can create anonymous blocks other than *D nnn (dimensions) and *X nnn (hatch patterns). Unreferenced anonymous blocks are purged from the BLOCK definition table when a drawing is opened. Referenced anonymous blocks (those that have been inserted) are not purged. You can use entmake to create a block reference (insert object) to an anonymous block. (You cannot pass an anonymous block to the INSERT command.) Also, you can use entmake to redefine the block. You can modify the entities in a block (but not the block object itself) with entmod.
The name (group 2) of an anonymous block created by AutoLISP or ObjectARX has the form *U nnn , where nnn is a number generated by AutoCAD. Also, the low-order bit of an anonymous block's block type flag (group 70) is set to 1. When entmake creates a block whose name begins with * and whose anonymous bit is set, AutoCAD treats this as an anonymous block and assigns it a name. Any characters following the * in the name string passed to entmake are ignored.
NoteAnonymous block names do not remain constant. Although a referenced anonymous block becomes permanent, the numeric portion of its name can change between drawing sessions.
