[Newbies] RectangleMorph vs. BlobMorph

[polishookm]

When I run following passages (first the code marked 1st, then the code 
marked 2nd, and last, the code marked 3rd),     1st: 100 RectangleMorphs 
draw to the screen
    2nd: they change position
    3rd: they disappear.

If I use BlobMorph instead of RectangleMorph, both Squeak and my 
computer freeze. Other than the obvious (don't do this with BlobMorph!), 
why is RectangleMorph ok in this circumstance whereas BlobMorph isn't.

Thanks in advance for all help ....

* * * * *

" 1st-----doit "
theBag := Bag new.
x := 10.
y := 10.
xIncrement := 4.
yIncrement := 4.
100 timesRepeat: [
   
    theMorph := RectangleMorph new.
    theBag add: theMorph.
   
    theMorph position: x at y.
    theMorph openInWorld.
   
    x := x + xIncrement.
    y := y + yIncrement.
].

" 2nd-----doit "
theBag do: [ :eachMorph |
    eachMorph position:
    (600 atRandom)@(600 atRandom)
].

" 3rd-----doit "
theBag do: [ :eachMorph | eachMorph delete].