Here is a list of available Descriptors for PowerBlocks. Below is how these descriptors will display in the builder.

custom_block 
color
checkbox
dimension
feed 
feed_field 
field_or_expr 
font_spec 
font_style 
gravity 
height 
html 
image 
key 
layer 
margin 
text 
text_box 
text_only 
transition 
url 
variable_name 
view 

Here is how these descriptors will look in the Block

POWER BLOCK ARTICLES:
- Building PowerBlocks
- Writing PowerBlock Descriptors