1. A non-transitory computer readable medium storing one or more sequences of instructions causing a digital processing system to facilitate designing of an integrated circuit containing a plurality of circuit blocks, wherein execution of said one or more sequences of instructions by one or more processors contained in said digital processing system causes said digital processing system to perform the actions of: receiving a corresponding model for each of said plurality of circuit blocks, wherein the model for a first circuit block contains timing information of timing arcs for a first set of ports of said first circuit block, and at least a portion of peripheral circuitry connected to a second set of ports of said first circuit block; and performing timing analysis of said first circuit block with at least one external block using said model of said first circuit block, said one external block and said first circuit block being contained in said plurality of circuit blocks; forming the model of said first circuit block by combining a first model and a second model of said first circuit block; wherein said first model is according to a black box model such that said first model contains timing information of timing arcs at ports of said circuit block, and wherein said second model is according to an interface timing model such that said second model contains at least a portion of circuitry connected to ports of said first circuit block, said forming further comprising: selecting said first set of ports from said first model, and said second set of ports from said second model, wherein the model for said first circuit block comprises said first set of ports with information corresponding to said first model and said second set of ports with information corresponding to said second model; wherein said combining comprises: instantiating said first model in said second model; deleting connections, formed by said instantiating, between ports of said first model and said second model that are determined to be said second set of ports, while retaining connections, formed by said instantiating, between ports of said first model and said second model that are determined to be said first set of ports; and deleting circuit portions of said second model corresponding to said first set of ports.