Black bear, the .45 with the right ammuntion like a bonded HP or cast bullet would be...well OK. The bullet weight is there, but velocity suffers in .45 ACP.
The 10mm on the other hand is adequate for defense against black bear and would be a reasonable choice over the .45 acp. What we have to understand is that most factory 10mm auto ammuntion is loaded down from the SAAMI specifications, putting it closer to the standard .357 magnum energy figures. When bullet weights of the 10mm and the .357 are compared the 10mm betters the .357 in almost all instances in energy and velocity, yet has more frontal area (an attribute to the venerable .45 ACP). Commercial Ammuntion from doubletap and buffalo Bore are at the higher end of the 10mm spectrum and have some impressive energy figures, especially when you put it in the slim and light weight profile of a semi-auto platform like the Glock pistol.
Handloaders of the 10mm, have the distinct advantage of capitolizing on premium bullets and powder and acheiving full performance of the 10mm cartridge without exceeding SAAMI pressures. I carry the Glock 20 afield for both 2 and 4 legged protection and deer hunting. I load up 180 GR Hornady XTP's (very good penetrating hunting bullet) to 1320 FPS. With 16 rounds at hand, I woudn't feel undergunned in a "chance" encounter with a Tennessee black bear.
If you don't handload or know someonw who does, look to doubletap or buffalo bore.
For me between the .45, .357 and the 10mm auto...Make mine the 10mm.